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 Ada Wong.