Alien

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.

Playing style
"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)

Weaknesses
"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.

Monster Cards

 * Alien Ammonite (Needed for Cosmic Fortress Gol'Gar. Can also make a instant Catastor.)
 * Alien Dog (Quickly generates A-Counters and field presence.)
 * Alien Hypno (Generates field presence.)
 * Alien Kid (One of the main A-Counter-generating engines)
 * Alien Mother (Has an effect similar to Goyo Guardian. Generates field advantage)
 * Alien Overlord (Easily Special Summoned. Generates A-Counters quickly.)
 * Alien Telepath (Is an extra Mystical Space Typhoon/Dust Tornado.)
 * Alien Warrior (Beatstick that generates A-Counters when destroyed)
 * Chaos Sorcerer
 * Cosmic Horror Gangi'el
 * Flying Saucer Muusik'i (Used for searching Aliens faster, although it has low stats for its Level)

Extra Deck

 * Cosmic Fortress Gol'gar (Can return to hand any number of face-up Spells/Traps to place same number of A-Counters on monsters, Once per turn can remove 2 A-Counters to destroy one card controlled by opponent)
 * Ancient Sacred Wyvern (Can get huge ATK boosts. Can revive itself)

Spell Cards

 * "A" Cell Breeding Device
 * "A" Cell Incubator
 * "A" Cell Scatter Burst
 * Burden of the Mighty (Helps keeping the usually low attack Aliens on par with powerful beatsticks. It can also be bounced with Cosmic Fortress Gol'gar.)
 * Code A Ancient Ruins
 * Mysterious Triangle
 * Nightmare's Steelcage (Gol'Gar can bounce to reuse and gain A-Counters)
 * One for One (For quickly summoning Alien Ammonite.)
 * Swords of Revealing Light (Gol'Gar can bounce to reset the turn count and gain A-Counters)
 * Viper's Rebirth

Trap Cards

 * Alien Brain
 * Brainwashing Beam
 * Cell Explosion Virus
 * Crop Circles
 * Damage = Reptile
 * Gravity Bind (Gol'Gar can bounce to reuse and gain A-Counters and cause a One-Sided affect limiting opponent from attacking)
 * Mass Hypnosis
 * Offering to the Snake Deity
 * Orbital Bombardment
 * Planet Pollutant Virus