|Appears in (sets)|
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 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. This is the first true Reptile archetype and one of the only archetypes, along with "Reptilianne", that focuses heavily on manipulating the opponent.
"Aliens" have a variety of lower-level monsters, such as "Alien Warrior" and "Alien Grey", that generate essential A-Counters. However, prior to "Crimson Crisis," they suffered from a distinct lack of effects to exploit these counters once they have been created. Cards like "Alien Telepath", "Alien Hypno", and "Alien Mars" have potentially disruptive, yet situational, effects. By contrast, the powerful field-clearing effect of "Cosmic Fortress Gol'gar", as well as the revival provided by "Code A Ancient Ruins", now give the deck a more reliable means of converting A-Counters into Card Advantage. "Crimson Crisis" also 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 newfound ability to Swarm the field.
"Crimson Crisis" and "Raging Battle" have indeed added a variety of needed support for "Alien" monsters, and as such there are now 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, such as "Brainwashing Beam" and "Mass Hypnosis". While such a strategy is still possible, it is distinctly suboptimal when compared to modernized builds that emphasize Gol'gar and the easy destruction and revival (through its quick counter generation for "Code A Ancient Ruins") that it supplies. 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 underpowered predecessor, "Cosmic Horror Gangi'el", together the so-called "boss monsters" of the "Aliens". However, the reptile-type "Evil Dragon Ananta" can have some amazing field clearing abilities. Neither of these monsters counts as an "Alien" per se, and hence does 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 said 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 opponent's monsters via Alien Hypno,Brainwashing Beam, and Mass Hypnosis require A-Counters to maintain control of the pilfered monsters.
Gol'gar control variants obviously exploit Gol'gar's ability both to generate A-Counters and to instantly convert those counters into free removal. There is a combo that uses Cosmic fortress Gol'gar and Swords of Revealing Light. By using Gol'gars 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 "Lightsworns","Zombie", or Blackwings. The deck itself has little draw power outside of Grey. which causes it to have trouble against faster decks like the ones above. 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.
"Aliens" also have no native, major offensive threat outside of Gol'gar. Although Warrior and Shocktrooper boast high ATK scores for Level 4 Monster Cards, they quickly lose value after the few first turns of the game, when Synchro Monsters, Fusion 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 those "speedy" decks. "Counter Cleaner" can make this deck useless if used correctly.
|Alien Ammonite||Allows you to instantly Synchro Summon for a Cosmic Fortress Gol'gar|
|Alien Mother||Is similar to Goyo Guardian, helping your swarm|
|Alien Overlord||One of the key cards of the deck, due to being easily summonable and fast to generate A-Counters.|
|Alien Warrior||A nice beatstick that helps generating A-Counters|
|Cosmic Horror Gangi'el|
|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 searcher for Alien Ammonite.|
|Shining Angel||Can search Ammonite, Kid and Dog easily while thinning your deck.|
|Cosmic Fortress Gol'gar|
|Light End Dragon|
|Ancient Sacred Wyvern||Great card to finish your opponent off and can be easily summoned with Ammonite + Overlord.|
|"A" Cell Breeding Device|
|"A" Cell Incubator|
|Brain Control||Steal a monster for Cosmic Horror Gangi'el.|
|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.|
|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. On your turn Gol'gar can bounce it so you can attack and gain A-Counters, then play it again before you end your turn.|
|One for One||For Special Summoning Alien Ammonite and possibly Alien Psychic|
|Solidarity||Gives a great ATK advantage and is good along side with Gol'Gar. However, it isn't recommended because it forces you to not running non-Reptile monsters, like Honest.|
|Swords of Revealing Light||Stops the opponent from attacking during their turn. On your turn Gol'gar can bounce it so you can attack and gain A-Counters, then play it again before you end your turn.|
|Viper's Rebirth||This card destroys your alien monster, so it can work with Code A Ancient Ruins.|
|Damage = Reptile|
|Detonator Circle "A"|
|Gravity Bind||Gol'Gar can bounce to reuse and gain A-Counters and cause a One-Sided affect limiting opponent from attacking|
|Offering to the Snake Deity||This card destroys your alien monster. So it works well with Code A Ancient Ruins.|
|Planet Pollutant Virus||Great field clearer that allows you to save yourself in a pinch.|