Wesker-Uroboros (Uroboros virus)
The once invincible, bullet-dodging Wesker deliberately infects himself with the Uroboros virus as a last resort after having been weakened by an overdose of the virus suppressant PG67A/W that was injected by Chris and Sheva. The Uroboros's characteristic tentacles have enveloped both arms, and the right arm has absorbed a variety of nearby metallic objects to form a particularly lethal weapon. Although Wesker has lost his lightning-fast movement and agility, his offensive and defensive abilities are more powerful than ever before.
RE5 was the first in the main series to offer campaign co-op. This is reflected in the design of the final boss battle where both players are initially forced to fight separately. As a side-note, the previous battle onboard the bomber is the only place in the main game where Wesker's weapon, the Samurai Edge, is clearly discernible (it also appears as part of Wesker's load out in the Mercenaries and Versus modes).