The game states that he is completely dead, but no one really knows. You do see his body float off into space. I mean, Sepiks was resurrected and killed again by us (re-buit), and the black heart was attempted to be brought back twice until we stopped it, and so our enemies being brought back is a possibility.
Sepiks wasn't brought back, and the Black Heart isn't a natural thing. Oryx is dead by our observations, which may be wrong. But like i said i believe he's dead.
Maybe it wasn't Sepiks, but a giant servitor was definitely brought back by the fallen, I forgot which mission or strike that was.
I do believe he is dead too, but we're not the developers, and things could change.
The Restorative Mind tried to bring Sekrion back as well (time travel). There might be other references that I'm missing, but it definitely shows that beings that we killed can be brought back in one form or another, story wise anyway.