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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Random thing I noticed in the mission. If you glitch to the top of the level in the final room there is an instant returning to allies field that has been added. Maybe if you do end up going back you can find some secret. I'll try to glitch back to the bridge and report my findings.