The Nyx is a large amoeba-like blob. It absorbs its enemies into its body with thing tentacles. It is unique amongst Umbrella B.O.W.s for its properties; i.e. assimilating other organisms to construct itself into much larger forms. However, it does share a similar trait to some of Umbrella's Tyrant projects, namely its core serves as its central nervous system akin to the exposed hearts of some Tyrants; i.e. a weak point of an otherwise near indestructible life-form.
It's worth noting that being absorbed by Nyx does not seem to be completely lethal. The heavily wounded Tyrant R was able to twitch and attempted to escape after being absorbed. And once Nyx was defeated and melted, the Tyrant remained physically intact and was able to recover and fight.