From Yu-Gi-Oh!

The Aliens are a series of Reptile-type cards 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 the use of "A-Counters", which weaken opposing monsters battling Aliens and allow the Alien user to take control of them. This is the first true Reptile archetype and one of the only archetypes that focuses heavily on manipulating the opponent.

[edit] Playing style

Aliens focus on manipulating the opponent rather than strengthening themselves. They have a wide range of lower-level monsters that can be used to oppress or manipulate the opponent but are lacking at higher levels; however, the ability to take control of an opponent's strong monster more than makes up for this deficiency. That being said, the new cards from "Crimson Crisis" have made aliens much stronger due to the powerful field-clearing effect of "Cosmic Fortress Gol'gar" and the rapid A-Counter distribution of "Alien Overlord", as well as the devastating force of "Planet Pollutant Virus", which wipes out any of your opponent's monsters without A-Counters and infects any they summon for the next three turns. And with the new addition of "Alien Dog" from "Raging Battle", Aliens are capable of swarming the field easily.

No monster in the Alien archetype has over 2600 ATK, and the two strongest monsters in the archetype ("Cosmic Horror Gangi'el" and "Cosmic Fortress Gol'gar", considered as the "Boss Monsters" of the Aliens) technically do not count as "Aliens", so they gain no personal benefit from the weakening effect of A-Counters during battle. An Alien user needs to be aware to rely as much upon their "weaker" monsters as their stronger ones in order to prevail.

However, Aliens do possess a relatively large number of low-level beatdown monsters ("Alien Warrior" and "Alien Shocktrooper"), each of which are powerful enough to devastate your opponent with little help. "Alien Warrior" in particular will weaken opponent monsters with A-Counters on them, effectively allowing it to destroy powerful cards such as Jinzo and the Monarchs; and, in the event that "Alien Warrior" is destroyed, will distribute a good number of A-Counters to ensure that whatever follows will have a distinct advantage.

"Code A Ancient Ruins" is easily abused with the effect of "Cosmic Fortress Gol'gar". By having multiple Code A Ancient Ruins and Cosmic Fortress Gol'gars on the field, you can use Gol'gar's effect to return the Ruins to your hand, producing more A-Counters and allowing you to use Ruins once more, which then lets you swarm the field quickly, or even Synchro Summon even more monsters by recycling "Alien Ammonite".

[edit] Weaknesses

The major weakness Alien decks have is that against decks that swarm can keep your Aliens from putting "A" Counters on them fast enough. Alien Kid, Alien Warrior, and Planet Pollutant Virus are a few ways to keep up with swarming strategies. They're also incredibly weak to "Skill Drain" decks, which will completely take away the field advantage Aliens need to have to abuse monsters with "A" counters.

[edit] A-Counters

Putting A-Counters on opposing cards is essential to the success of Alien decks, as Aliens generally have no extremely powerful monsters other than "Cosmic Horror Gangi'el" and "Cosmic Fortress Gol'gar".

Since counters cannot be placed on face-down monsters, generally the best way to ensure that A-Counters can be used against the opponent is to use "Light of Intervention", though note that doing so negates much of the usefulness of "Alien Grey". Another use for "Light of Intervention" is that when Tribute Summoning "Flying Saucer Muusik'i" it can be Summoned in face-up defense position, creating a temporary barrier and allowing you to use its effect during your next Draw Phase. Other alternatives include "Ceasefire" or "Swords of Revealing Light".

According to the current ruling on A-Counters, they do not reduce the ATK and DEF of monsters battling Alien monsters if no cards exist on the field that mention that effect. This means that if you only control an "Alien Shocktrooper", for example, your opponent's monsters will not be weakened, no matter how many A-Counters are on them. The upside to this ruling is that these effects accumulate, so if you control two "Alien Warriors", the opponent's monsters will be twice as weakened for every A-Counter on them.

Aliens
Members
TCG/OCG	Alien Ammonite ● Alien Dog ● Alien Grey ● Alien Hunter ● Alien Hypno ● Alien Infiltrator ● Alien Kid ● Alien Mars ● Alien Mother ● Alien Overlord ● Alien Psychic ● Alien Shocktrooper ● Alien Skull ● Alien Telepath ● Alien Warrior
Anime/Manga
Support
TCG/OCG	Alien Ammonite ● Alien Dog ● Alien Grey ● Alien Overlord ● Alien Skull ● Alien Warrior ● Cosmic Fortress Gol'gar ● Cosmic Horror Gangi'el ● Flying Saucer Muusik'i ● Interplanetary Invader "A" "A" Cell Breeding Device ● "A" Cell Incubator ● "A" Cell Scatter Burst ● Brainwashing Beam ● Cell Explosion Virus ● Code A Ancient Ruins ● Corruption Cell "A" ● Crop Circles ● Detonator Circle "A" ● Interdimensional Warp ● Mass Hypnosis ● Mysterious Triangle ● Orbital Bombardment ● Otherworld - The "A" Zone ● Planet Pollutant Virus
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Related
TCG/OCG
Anime/Manga