HAOS was a massive, human-based B.O.W. created with the C-Virus. It was encountered by B.S.A.A. operatives Chris Redfield and Piers Nivans in an underwater facility off the Chinese coast. Its name comes from the Serbian word for "chaos".
The HAOS project was Carla Radames' and Neo Umbrella's ultimate bio-weapon, and was so important to Carla's plans that it required an entire underwater facility to support the creature once its creation was complete.
Carla's plan was to unleash HAOS to spread the C-Virus across the entire world, where it would agamogenesize and disperse the virus in a gaseous form in a method comparable to the Lepotitsa, only with a higher concentration of the virus. Neo Umbrella planned to release Haos once it had fully developed but a contingency plan -Plan D- was devised in case Carla was killed before the B.O.W was complete. The creature would be released regardless of whether it was fully developed, with the intention that it could spread the virus across the globe before its imperfections took effect.
When Chris and Piers infiltrated the facility to rescue Sherry Birkin and Jake Muller, they discovered a colossal chrysalid containing HAOS, which hatched while they were trying to evacuate. The massive B.O.W. chased them around the chamber it was sealed in until they retreated to an elevator. Haos continued to pursue the two BSAA agents and eventually cornered them in a large flooded room, where they rendered it unconscious with heavy weaponry.
With the facility badly damaged by HAOS' rampage, Chris and Piers attempted to reach the escape pods but the B.O.W., which had now escaped into the ocean, remained determined to kill them. After an intense chase through a tunnel, HAOS trapped them in another large chamber. The creature attacked Piers and threw him across the room where his arm was crushed under falling debris. Haos then seized Chris but was heavily injured when Piers injected himself with the advanced strain of the C-virus and used his mutated arm to electrocute the beast, which then encased itself in a chrysalid.
Now fully regenerated, Haos broke out of its chrysalid and attacked the two agents. However, its decreased size and vulnerability to Piers' electrical attacks enabled Chris to damage its vulnerable organs which badly injured the creature. It made a final attempt to kill Piers, which Piers tried to deter by impaling the creature with his mutated arm, but Chris was able to rescue him and the two of them destroyed the B.O.W.s heart, supposedly killing it.
However, the incredible resilience of HAOS enabled it to survive the battle and it made one final attack on Chris after Piers had ejected him from the facility in an escape pod. The B.O.W attempted to destroy the pod with Chris inside it but was forced off by another electrical blast which most likely came from Piers. HAOS was finally killed when it was consumed in the explosion of the underwater facility.