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 Ruins. "Alien" decks that manipulate opponent's monsters via Hypno, Beam, and 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 your opponent controls.(you can only use this combo once per-turn)

Some "Alien" cards have an effect that limits monsters with A-Counters on them.
 * "Alien" Effect A - (If a monster with an A-Counter battles an "Alien" monster, it loses 300 ATK and DEF for each A-Counter during damage calculation only.)
 * Alien Kid, Alien Grey, Alien Overlord, Alien Telepath, Alien Warrior, Cosmic Horror Gangi'el
 * This ablity can stack. Two monsters with this effect on the field will cause 600 Att/Def lost per A-Counter.
 * "Alien" Effect B - The effects of Effect Monsters with A-Counters are negated.
 * Alien Mars
 * "Alien" Effect C - Monsters with A-Counters cannot declare an attack.
 * Alien Psychic

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. Accordingly, it often suffers in its quest to draw into its power cards and convert them into a superior setup before losing the race to 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 massive 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 more explosive decks. "Counter Cleaner" can make this deck useless if used correctly.

Monster Cards

 * Alien Ammonite (LVL1, LIGHT-Attribute, Tuner, Can Special Summon a one LVL 4 or lower "Alien" monster upon Normal Summon - destroyed upon End Phase)
 * Alien Dog (LVL3, LIGHT-Attribute, When Normal Summoning an "Alien" can Special Summon this card from hand, When Special Summoned by its own effect it can place 2 A-Counters on opponent's monsters)
 * Alien Grey (LVL2, LIGHT-Attribute, FLIP: place a A-Counter on an opponent's monster, When destroyed by battle after flip draw 1 card)
 * Alien Hypno (LVL4, Gemini, Once per turn can steal one opponent monster that has A-Counters on it)
 * Alien Kid (LVL4, LIGHT-Attribute, Special Summoned monsters onto opponent's side gain 1 A-Counter upon summon, "Alien" Effect A)
 * Alien Mars (LVL3, "Alien" Effect B)
 * Alien Mother (LVL6, DARK-Attribute, Special Summons monsters with A-Counters that this card destroys at end of Battle Phase)
 * Alien Overlord (LVL6, DARK-Attribute, Can remove 2 A-Counters to Special Summon from hand, Can place 1 A-Counter on each monster on opponent's field once per turn, "Alien" Effect A)
 * Alien Psychic (LVL1, DARK-Attribute, Changed to Defence upon Normal/Flip Summon, "Alien" Effect C)
 * Alien Shocktrooper (LVL4, 1900Att, No effect)
 * Alien Skull (LVL4, Can tribute one LVL 3 or lower monster on opponent's field to Special Summon to opponent' field by losing ability to Normal Summon/Set that turn, "Alien" Effect A)
 * Alien Telepath (LVL4, Once per turn can remove 1 A-Counter to destroy 1 Spell/Trap on field, "Alien" Effect A)
 * Alien Warrior (LVL4, When destroyed by battle adds 2 A-Counters to monster that destroyed Alien Warrior, "Alien" Effect A)
 * Chaos Sorcerer (LVL6, DARK-Attribute, Staple card, Can remove from play one face-up monster instead of attacking once per turn)
 * Cosmic Horror Gangi'el (LVL7, LIGHT-Attribute, Can tribute one monster origionally owned by opponent to Tribute Summon, Can place 1 A-Counter on one opponent monster once per turn, "Alien" Effect A)
 * Flying Saucer Muusik'i (LVL5, LIGHT-Attribute, Can add one "Alien" card to hand instead of drawing during Draw Phase)
 * Interplanetary Invader "A" (LVL1, LIGHT-Attribute, Gain control of a monster that attacks this face-up Attack Position card, at the end of the Battle Phase)
 * Oshaleon (LVL3, If Attack then opponent can only attack "Oshaleon", When destroyed by battle can add a Reptile monster, with ATK equal or less than 500, to your hand)
 * Alien Ammonite, Alien Grey, Alien Psychic

Extra Deck

 * Ally of Justice Catastor (LVL5, DARK-Attribute, Destroys any Non-DARK monster that battles with this card)
 * Armory Arm (LVL4, LIGHT-Attribute, equips to other monsters to increase Att by 1000 and deals damage to opponent equal to Att of destroyed by battle monsters)
 * Cosmic Fortress Gol'gar (LVL5, LIGHT-Attribute, Requires Alien Ammonite, 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 controled by opponent)

Spell Cards

 * "A" Cell Breeding Device
 * "A" Cell Incubator
 * "A" Cell Scatter Burst
 * Brain Control (Steal a monster for Cosmic Horror Gangi'el)
 * Code A Ancient Ruins
 * Mind Control (Steal a monster for Cosmic Horror Gangi'el)
 * Molting Escape (Equip to Reptile-Type, can prevent destructon by battle once per turn, increases ATK of protected monster by 300 each time it protects equiped, Gol'Gar can bounce to reuse on another "Alien" and gain A-Counters)
 * Mysterious Triangle
 * Nightmare's Steelcage (Gol'Gar can bounce to reuse and gain A-Counters)
 * One for One (For Alien Ammonite and lesser extent Alien Psychic)
 * Swords of Revealing Light (Gol'Gar can bounce to reuse and gain A-Counters)
 * Viper's Rebirth

Trap Cards

 * Alien Brain
 * Bottomless Trap Hole
 * Brainwashing Beam
 * Cell Explosion Virus
 * Crop Circles
 * 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)
 * Mass Hypnosis
 * Mirror Force
 * Offering to the Snake Deity
 * Orbital Bombardment
 * Planet Pollutant Virus
 * Snake Whistle
 * Torrential Tribute