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.
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. Some "Alien" monsters resemble the stereotypical Grey aliens, while others are more reptilian in appearance.
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 the effect of "Gol'gar".
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 the effect of "Gol'gar" 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 the effect of "Gol'gar" 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 Card 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.