I managed to kill one of the Ir Yuts and then the invisibility wore off, but then came back. tried to kill the second but got detected. managed to kill the first a few times, but couldn't manage to kill the second without being detected. I think there's a way to kill both without being detected and then steal the essence. The ultras may come into play at the end along with the adds, but haven't gotten that far.
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I bet killing Ir Anuk, Ir Halak and the 2 Ir Yuts then killing Baxx (or whatever his name is) and the taken centurion/colossus maybe.