Nyx

Named after the Greek goddess of the night (and interestingly, the mother of Thanatos - the Greek god of Death) was a B.O.W. (Bio-Organic Weapon) developed by Umbrella Corporation around the time of the t-Virus incident which plagued Raccoon City in September 1998.

It's uncertain exactly what the purpose of Nyx was but it was most likely developed as a research tool as it had the ability to absorb any biological material and assimilate it into its own body (including other extremely powerful B.O.W.s such as a Super Tyrant). It's not known if at some point bodies become fully 'processed' by the body and simply become one with it or remain attached in some way - this is purely hypothesised as the only time the Nyx was encountered it had a cluster of humans attached to one arm at varying levels of absorption.

The Nyx was held in a container at an Umbrella shipping yard, presumably in deep storage. However, it was stolen by a rogue U.S.S. (Umbrella Security Service) captain and flown over the city by helicopter. Depending on the scenario, the container can either leave the city safely or it can be damaged near the Raccoon City overpass which causes the creature to fall out onto the ground below and activate.

When active, it stomps around looking for material to absorb. Visually, it resembles a walking sphincter (it really does) and has a series of tentacles which it can use to draw in its prey. When the tentacles make contact, it supplies a constant t-Virus infection which presumably is its preferred 'flavouring'. It can also swing its huge arm like a club if necessary.

Interestingly, when a severely injured Tyrant R in its Super Tyrant state stumbled in its path, it attempted to absorb it. When absorbed, it took place on the left side of its body. Initially the incorporation seemed to be successful but eventually the Tyrant had enough of its strength restored to fight back (perhaps gained from Nyx itself, inadvertently) resulting in some twitchy movements.

When the Nyx was eventually defeated, it broke down into a pile of steaming biomass, leaving a fully intact Tyrant R in its place. The Tyrant R then resumed the attack (and it was eventually killed sometime after).