"Alien Grey" in the artwork of "Brainwashing Beam"
They were introduced in Power of the Duelist, with further support added in Cyberdark Impact, Strike of Neos, Force of the Breaker, Tactical Evolution, Gladiator's Assault, Crimson Crisis and Raging Battle.
"Alien" Decks focus heavily on exploiting "A-Counters", which weaken opposing monsters battling against "Aliens" and permit "Alien" users to activate a variety of specific theft, revival, and destruction effects.
They seem to be based on many science-fiction views of aliens, and the way they operate in stories, making use of such tropes as brainwashing, genetic manipulation, and biological experimentation (as evidenced by "Interplanetary Invader "A"", which appears to be a mutated, living, or even intelligent form of an A-Counter). Their appearance seems to draw inspiration from these as well, like big eyes, tails instead of feet, large claws, long tentacles, slender bodies, multiple arms, etc.
The archetype's name comes from the term: "Space Alien", an extraterrestrial being from outer space.
"Aliens" have a variety of lower-level monsters, such as "Alien Warrior" and "Alien Grey", that generate those essential A-Counters. However, prior to "Crimson Crisis," they suffered from a distinct lack of effects to exploit these counters once created. Cards like "Alien Telepath", "Alien Hypno", and "Alien Mars" have potentially disruptive, yet situational, effects. The powerful field-clearing effect of "Cosmic Fortress Gol'gar", as well as the revival engine provided by "Code A Ancient Ruins", lends the Deck a more reliable means of converting A-Counters into Card Advantage. "Crimson Crisis" introduced "Planet Pollutant Virus", a card which doubles as mass-removal and lingering A-Counter generation. Cards like "Alien Overlord" from "Crimson Crisis" and "Alien Dog" from the subsequent "Raging Battle" also gave "Aliens" a bit of Swarming power.
There are several different builds and play styles for the archetype. The "Alien" cards prior to "Crimson Crisis" emphasized lower-level effect monsters like "Alien Psychic" and "Alien Hunter", and monster manipulation in the form of "Brainwashing Beam" and "Mass Hypnosis". A viable strategy, it is distinctly suboptimal when compared to modernized builds that emphasize "Gol'gar" and the easy destruction and revival it supplies through its quick counter generation for "Code A Ancient Ruins." "Gol'gar" can further be paired with powerful Continuous Spell Cards like "Prohibition" or "Burden of the Mighty", along with cards like "Ancient Forest" and "Swords of Revealing Light," to create a strong Control archetype that excels at A-Counter generation via Gol'gar's effect.
No monster in the archetype has over 2600 ATK, a value shared by "Gol'gar" and its underrated predecessor, "Cosmic Horror Gangi'el", together the so-called "boss monsters" of the "Aliens". The Reptile-type "Evil Dragon Ananta" is often included as a third "boss monster" and can often turn the tide of a duel via its field clearing abilities. None of these monsters count as "Alien" monsters, per se, and hence do not weaken opposing monsters burdened with A-Counters. This drawback is largely irrelevant, however, especially in the case of "Gol'gar", which is the highest ATK Level 5 Monster Card in the game and able to eliminate most threats from the field either by its effect or its disproportionately high ATK.
All "Alien" Decks ultimately rely on A-Counters to operate. A swarming "Alien" Deck needs ample A-Counters to Special Summon "Overlord" and activate "Code A Ancient Ruins". "Alien" Decks that manipulate the opponent's monsters via "Alien Hypno", "Brainwashing Beam", and "Mass Hypnosis" require A-Counters to maintain control of pilfered monsters. Another important effect of an "Alien" Deck is the A-Counter's ability to reduce the ATK and DEF of a monster by 300 points for every counter on it. The effect reduces the ATK of Monsters fighting "Aliens" for each "Alien" with that effect on the field. More easily explained, (Number of monsters with ability) X (Number of counters on monster fighting an "Alien") X 300 = Amount of ATK lost. Most "Alien" monsters have this effect built in. Some "Aliens" with powerful effects, like "Alien Hypno" and "Alien Dog", do not.
Gol'gar control variants obviously exploit "Gol'gar's" ability to both generate A-Counters and to instantly convert those counters into free removal. A popular combo involves "Cosmic Fortress Gol'gar" and "Swords of Revealing Light": By using "Gol'gar's" ability to return face up Spell and Trap cards of your choosing back to the owner's hand, you can return your "Swords of Revealing Light" back to your hand and play it back on the field. By repeating this you'll have unlimited "Swords of Revealing Light". Also you'll put A-counters on all face up monsters on the field including your own, and destroy pesky cards that your opponent controls (you can only use this combo once per-turn).
"Aliens" are weak against Decks with superior speed, such as "X-Saber","Legendary Six Samurai", or "Blackwing". The Deck itself has little draw power outside of "Grey", which causes it to have trouble against faster Decks. Specialized searchers such as "Gold Sarcophagus", "Oshaleon", "Snake Whistle", and "Damage = Reptile" can partially alleviate this problem by letting you retrieve your combo pieces more efficiently, but not all of them fit comfortably into the archetype and many of them can be situational as draws. You can also add "Pot of Duality" for more consistency to the deck.
The deck is also weak against "Fire King" Decks, as Fire Kings rely upon the destruction of cards and Special Summoning from the Graveyard, as well as clearing the field of monsters and Spells, which can force Aliens to stall. It is difficult for Aliens to overcome a card with high ATK and no DEF.
The "Alien"-based Deck is also bad against "Gladiator Beast", as they constantly tag out, preventing A-Counters from piling up. In this matchup, Aliens have to rely on pure power, which is not generally their focus. You can put a "G.B. Hunter" in your Side Deck to counter this.
"Aliens" aren't much use against "Ghostrick" decks, as most Ghostricks have the ability to flip themselves facedown, clearing away any counters on them and preventing more from being added. Additionally, their low attack power means it will be difficult to do damage even while a Ghostrick Field Spell is active.
"Aliens" also have no native, major offensive threat outside of "Gol'gar". Although "Warrior" and "Shocktrooper" boast high ATK for Level 4 Monster Cards, they quickly lose value after the few first turns of the game, when Fusion Monsters, Synchro Monsters, Xyz Monsters and other powerful cards begin hitting the field. Gol'gar helps to keep "Aliens" in the race. Other Synchros, like "Ally of Justice Catastor" (instantly Summonable by "Ammonite" and useful against "Lightsworns" in particular), also help the Deck to keep a solid footing against speedy Decks. "Counter Cleaner" and "Enervating Mist" can also seriously hamper "Alien" Decks if used correctly due to their high reliance on A-Counters, so add cards like "Dark Bribe" to negate them.
|Alien Ammonite||Allows you to instantly Synchro Summon for a "Cosmic Fortress Gol'gar" by Special Summoning any level 4 "Alien" from your Graveyard.|
|Alien Dog||Gives you the ability to quickly control the field. By summoning "Warrior" and Special Summoning this card allows you to have two counters to summon "Alien Overlord". Then, by playing a "Code A Ancient Ruins", your "Aliens" are almost unstoppable, apart from mass monster removal. You can also Special Summon in response to "Alien Ammonite" for a level 4-8 Synchro Monster.|
|Alien Grey||This card adds only one A-Counter, however it also grants draw power that is missing in this Deck type, drawing an extra card.|
|Alien Hypno||Special Summon it with "Mysterious Triangle" or "Code A Ancient Ruins" and you can Gemini Summon it. If you played "Planet Pollutant Virus" before reviving "Hypno", you could take control of all your opponent's monsters.|
|Alien Kid||Helps when an opponent swarms the field, especially when combined with "Mass Hypnosis".|
|Alien Mars||This card negates the effects of monsters with A-Counters, so it can be used as a "Skill Drain" on your opponent's monsters. Doesn't negate the activation so cards like "Stardust Dragon" will still work. Can be used with "Alien Ammonite" to make "Armory Arm".|
|Alien Mother||Is similar to "Goyo Guardian", helping your swarm but lacks the A-Counter 300 effect, making it hard to use against monsters stronger than itself.|
|Alien Overlord||One of the key cards of the Deck, due to being easily summonable and fast to generate A-Counters.|
|Alien Psychic||Useful by having its ability, being a DARK-Type for "Chaos Sorcerer", and can be used with "Crop Circles" to pull "Alien Ammonite" out easier. Combined with a card like "Overlord" or "Gang'iel" you can maintain control over the field and stop all summoned monsters from attacking. With a "Sakuretsu Armor" or "Magic Cylinder" in case of swarming, it gives you a good control of the field.|
|Alien Telepath||Handy for Spell and Trap removal.|
|Alien Warrior||A nice beatstick that helps generating A-Counters. Its effect allows you to Special Summon it from the Graveyard with "Code A Ancient Ruins" after it has been destroyed by battle.|
|Black Luster Soldier - Envoy of the Beginning||Many strong "Aliens" are LIGHT and DARK attribute, making this a surprising trump card. This is a little better than "Chaos Sorcerer" (below) due to the same effect with optional double attack in exchange for not using the effect.|
|Chaos Sorcerer||Many strong "Aliens" are LIGHT and DARK attribute, making this a surprising trump card. Capable of removing troubling Monsters without having to attack them as well as banish them if they are a searcher like "Pyramid Turtle".|
|Cosmic Horror Gangi'el||A nice alternative to "Overlord", which can generate A-Counters each turn giving you good command of the battlefield, and is only held back with its high level. Combining with either "Brainwashing Beam", "Alien Mother", or "Alien Hypno", it is easily summonable however.|
|Evil Dragon Ananta||Powerful either as a finisher move or as a last resort.|
|Honest||Quite a large amount of "Aliens" are LIGHT attribute. Along with the ATK reduction effects, this card can provide extra protection against strong monsters.|
|Oshaleon||A possible searcher for "Alien Ammonite".|
|Reptilianne Naga||A Reptile-Type Monster that can reduce other Monsters to 0 attack and it cannot be destroyed by battle. You can also use it as an extra Synchro Material Monster.|
|Rescue Rabbit||Because Rabbit banishes itself (if it is not destroyed) you can still use Solidarity(provided your Xyz is also Reptile-Type) or other cards which rely on having reptiles exclusively in the Graveyard, such as Alien Ammonite. Can be used with Alien Shocktroopers to make any Rank 4 Xyz, particularly King of the Feral Imps, which can detatch a Material to search for said Alien Ammonite. Because Shocktrooper is in the grave, you can make a relatively quick Cosmic Fortress Gol'gar or other Level 5 Synchro next turn.|
|Shining Angel||Can search "Alien Ammonite" and "Alien Dog" easily while thinning your Deck.|
|Cosmic Fortress Gol'gar||The best generating card in this Archetype, with the best attack of all the Level 5 Monsters, and being easily synchro summoned by "Alien Ammonite", you can gain control almost instantly by summoning "Ammonite" and dumping an "Alien" to the Graveyard with "Lightning Vortex" or "Raigeki Break". Also, combined with "Swords of Revealing Light", you can generate A-Counters without being attacked. Can additionally bounce your opponent's Spells and Traps, causing some effects to not affect you properly.|
|Ally of Justice Catastor||A very powerful Synchro that can help stall for a finishing move. Can also be summoned easily with "Alien Ammonite" and a Level 4 "Alien" Monster.|
|Ancient Sacred Wyvern||Great card to finish your opponent off and can be easily summoned with "Ammonite" and "Overlord".|
|Armory Arm||Easily Synchro Summoned using "Alien Ammonite" and "Alien Mars" or "Alien Dog". Adds serious Effect Damage to "Gol'gar" or a stolen opponent's monster along with a huge boost in attack.|
|King of the Feral Imps||Can search any reptile-type monster from the deck.|
|Light End Dragon||A good card to take out bigger monsters.|
|Scrap Dragon||Easily Synchro Summoned using "Alien Ammonite" with other "Aliens" and useful to destroy your opponent's cards.|
|"A" Cell Breeding Device||Steady way for dispensing A-Counters.|
|"A" Cell Incubator||Take counters off this card to effectively use effects for free. Each time an effect removes an A-Counter(s), you can add up to 3 more to the Field (one per ""A" Cell Incubator" on the Field). Can be destroyed using "Alien Telepath" in order to distribute the A-Counters.|
|"A" Cell Scatter Burst||A quick way to place A-Counters on the field. Destroys your "Alien" Monster and places A-Counters equal to that Monster's level on any Monster your opponent has. If you have "Code A Ancient Ruins" on the field, a level 4 Alien equals 4 A-Counters, and you can resummon your monster again, works well with "Mass Hypnosis", placing A-Counters on as many cards as possible. Also, due to to the large amount of A-Counters that it creates, it can be used with basically any A-Counter using card.|
|Burden of the Mighty||Very powerful continuous spell that helps keep the usually low attack "Aliens" on par with powerful beatsticks. It can also be bounced with "Cosmic Fortress Gol'gar".|
|Code A Ancient Ruins||You can use this card's effect, then return it to your hand with "Gol'gar". You can then play this card again to Special Summon a second "Alien" from your Graveyard because returning this card to your hand bypasses the "Once per turn" limit.|
|Molting Escape||Equips to all "Aliens", prevents battle destruction once per turn, "Gol'Gar" can bounce to reuse on another "Alien" and gain A-Counters.|
|Mysterious Triangle||This card destroys your "Alien" Monster as well, so it can work with "Code A Ancient Ruins". It also works well with "Gol'gar", clear the opponent's field, swarm and attack for game.|
|Nightmare's Steelcage||Stops the opponent from attacking during their turn. "Gol'gar" can bounce it so you can attack and gain A-Counters; play it again before you end your turn.|
|One for One||For Special Summoning "Alien Ammonite" and possibly "Alien Psychic".|
|Otherworld - The "A" Zone||Guarantees an extra ATK and DEF loss: your weaker "Alien"s can survive longer. It is also bounceable with "Gol'Gar".|
|Solidarity||Gives a great ATK advantage and is good along side with "Gol'Gar". However, it isn't recommended because it forces you to abandon using non-Reptile monsters, like "Honest" or "Chaos Sorcerer".|
|Swords of Revealing Light||Prevents your opponent from attacking. "Gol'gar" can bounce it so you can gain A-Counters; play it again before you end your turn.|
|Viper's Rebirth||This card destroys your "Alien" monster; it can work with "Code A Ancient Ruins".|
|Pot of Duality||Adds consistency to the deck. This card can be a better use card than "Alien Grey".|
|Alien Brain||Use to take control of your opponent's strongest monster when they attack, for turning the tides or summoning "Gang'iel".|
|Brainwashing Beam||Useful to steal the only monster your opponent has. Unloading an ""A" Cell Incubator" on the targeted monster will keep it yours for a while. Will also allow you to have opponent's monsters for tribute and Synchro Summon. This card can also be bounced with "Cosmic Fortress Gol'gar" to switch monsters.|
|Cell Explosion Virus||A "Mirror Force" with a slight requirement that is easily met through this Deck. Nasty response when your opponent thinks that your end is near. Much less useful now that "Mirror Force" is Unlimited.|
|Crop Circles||This nifty card can trade in any monster on your side of the field (including opponent's monsters you have taken) for an "Alien", you can tribute "Psychic" for "Ammonite", "Mother" for "Overlord" and "Skull" for "Warrior". If your opponent attempts to use "Dimensional Prison" or "Mirror Force" you can use this card, which will stop the effect. You can also trade in multiple cards to combine their levels, such as "Alien Dog" and "Mars" for "Overlord" or other combinations.|
|Damage = Reptile||A good card for swarming or replacing "Aliens" on the field. When a weak reptile monster gets killed, you can bring out a monster with attack less then the damage you took. Difficult to use as most "Aliens" have 1600 ATK with the except of "Alien Dog" (whose effect is useless through this card), "Alien Mars" (useful but situational), and weaker cards like "Ammonite" or "Alien Psychic" and "Gray".|
|Detonator Circle "A"||Can counter an opponents attack, but has a double edged effect.|
|Gravity Bind||"Gol'Gar" can bounce this to reuse, gain A-Counters, and cause a One-Sided affect limiting opponent from attacking.|
|Light of Intervention||You can't place counters on face-down monsters, which this card prevents. This card can also be bounced with "Gol'Gar".|
|Mass Hypnosis||A great card. Use with "Alien Overlord's" effect and you can gain all your opponents monsters. Acts like a temporary "Alien Hypno" without having to do any summoning. Bounce it with "Gol'gar" to allow control during each of your turns.|
|Offering to the Snake Deity||This card destroys your "Alien" Monster, making it work well with "Code A Ancient Ruins". If you have two "Ancient Ruins" face-up on the field, you can instantly bring the "Alien" you destroyed back. This also puts "Aliens" in the Graveyard for "Alien Ammonite's" effect.|
|Planet Pollutant Virus||Great field clearer that allows you to save yourself in a pinch. Places counters on monster summoned for the next three turns, and combined with a control trap, summoned monsters are quickly taken away.|
|W Nebula Meteorite||Easy way to bring "Cosmic Horror Gangi'el" onto the field.|
|Call of the Haunted||Revives your monsters and can be bounced back by Gol'gar's effect to use again.|
|Fiendish Chain||Negates monster effects that can hurt your strategies or help their strategies and can be bounced back by Gol'gar's effect to use again later.|