The Vaults of Atal'Utek
Oppose the Foes is an achievement in the Vaults of Atal'Utek that requires players to defeat all 3 Ancient Foes: Congealed Malice, Khu'tulak, and Susarikk. Only one Ancient Foe is available per week and rotates, so this achievement takes three weeks minimum. If you've missed a previous week, you'll have to wait until a future week when the missing boss comes back into rotation.
Ancient Foes are directly available after killing a Temple Incursion and will show up on your minimap. Defeating Ancient Foes grants Corrosive Coin and also a Corrosive Soul and Venom-Cursed Fragment for your first kill of the week. Further kills each week keep dropping the Venom-Drenched Sack as well, so it's worthwhile to tag them each time they're up.
This achievement is one of the many achievements required for the Meta Assault the Vault, which awards the Venomous Coiler.
