The Rank in Hell power grants the benefits of demonhood/vassalship to non-vassaled Revenants.
You absorb the power of the Dark Oppressor and gain some of their dominion over Stygia. That infernal plane will treat you as if you belong there, regardless of your morality.
With the recent targeting change to stygian pets, DO powered Revs are targeted by stygian pets. I feel this shouldn't be the case. The Dominion over Stygia imbued to such a Rev should come with respect from Imps and Hounds. It gives planar protection and the demon search bonus, why not protection from feral pets?
No Respect! No Respect!