Aliens are an archetype that has been introduced through the Main Box Abyss Encounters.
Aliens use A-Counters:
These counters are used to activate card effects from monsters, traps and spells. Some Aliens are able to use A-counters to reduce enemy monsters attack and defense, and will have the text (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.) This effect stacks for each Alien monster on the field. Aliens have been labeled as the best F2P and new player friendly Deck in Duel Links with only one core Super Rare card. Alien decks are extremely versitile and have room for situational tech cards, making them a strong deck for experienced players as well. I'm an experienced Alien player and have won the (9th) Meta Weekly for those interested in my alien resume.
Viable Alien Cards
Synergistic Non-Alien Cards (Useful Tech)
Core Card Breakdown
Run 1-2 copies of this card. 2 copies can brick without A-counters or tribute fodder, but one copy can feel like too few of your best “A-counter” spreading monster. Overlord can put out A-counters to each face up monster your opponent controls each turn. You can special summon Overlord by removing 2 A-counters anywhere. This card is versatile, but a potential brick without the means to summon it. During mirror matches, your first priority should be removing A-counters from your own monsters by special summoning your own Overlord. This way, your opponent cannot activate cards such as Mysterious Triangle, Alien Telepath, or Brainwashing Beam and also can not make use of the 300 attack and defense reductions from A-counters.
This card is played from 2 copies to 3. Alien Telepath is the main spell and trap removal of the deck, as it removes 1 A-counter from your opponent's monster to destroy a backrow. This effect can only be activated once per turn (per Alien Telepath), and it is worth noting that it does not negate. Your opponent has the opportunity to chain anything that they can.
This card is played from 2 copies to 3. It is an 1800 beater that places 2 A-counters when destroyed by battle on the monster that destroyed it. This is a consistent source of counters, and 2 just so happens to be the amount of counters required to summon Alien Overlord.