The main antagonists of the game. XCOM Enemy Unknown has made a plot point of the diversity in aliens that are under the elders control and XCOM2 continues that tradition with the alien design escalating more and more as the campaign goes on.
A Better ADVENT introduces variants of the alien races that split them off into various combat or utility roles and tunes them towards different points in the campaign so the alien races don’t become irrelevant at some point.
Here are some examples for alien variants introduced by A Better ADVENT:
Sectoids: New Sectoid variants put more or less emphasis on their psi abilities, depending on the unit. The Sectoid Soldier is almost without any psi, instead acting like a hyper aggressive flanker in combat. On the other hand, there’s the Mindbender who has powerful mindcontrol at his disposal while being immune to disorientation himself. The Puppeteer Sectoid is able to raise an army of Psi Zombies that will even continue to live on if the Puppeteer gets disabled or killed.
Vipers: The Viper variants are very different from each other. The Wyvern spits fire, torching large areas. The Boa is a massive tanky melee snake that will only let go of bound targets once its near death. And with the Mamba the aliens have a sniper viper on their team that can use a Boltcaster at squadsight range.
Mutons: The Mutons introduced by A Better ADVENT are mostly different in what equipment they use. The Muton Infector spreads Poison with its weapon. Meanwhile the Muton Pyro sets enemies hit by its energy rifle on fire.
Berserker: The Firestarter is a particularly unique and imposing unit added to the alien roster by A Better ADVENT. This Berserker is able to breath fire in a wide cone in front of him. If the Berserker is the alien’s battering ram, then this is a battering ram with a flamethrower built in.
Archons: The Valkyrie is a female version of the Archon. While she is not able to shoot at enemies from afar, she is an expert in melee combat. Her combat staff has the terrifying property of healing the Valkyrie whenever she hits an enemy. Sentinal Archons are guards. They heavily rely on Overwatch and associated perks like Guardian or Covering Fire. Meeting a group of Sentinels that immediately go on Overwatch once triggered will lead to a standoff situation that XCOM will need to break carefully.
Chryssalids: Chryssalids gain a major expansion with A Better ADVENT, introducing versions of them in many stages of the game. While at first XCOM might only encounter the lowly Crawler, barely even bigger than a cat, this will soon escalate until there is all kinds of bugs roaming around, from Chryssalid Bleeders to Hunterkillers. Once a certain point has been reached, even the Chryssalid Queen and King might come out to play.