I think for me it would be killing Cabal. Have you seen what they look like under their helmets in the videos? They are uglier then Emperor Palpatine.
What enemy alien race do you want to obliterate first?
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i want slay the hive because i heard they looked like zombies(sorta) and we can't have that. and i heard they had souls pop out of them when they died and that sounds like a cool death animation