Dark Sector is an action game, in which the player takes on the role of Hayden Tenno , an elite black-ops agent, sent into Lasria, a small USSR country, tasked with destroying a virus that turns people into mutants and eliminating its creator, Mezner. The mission is to find and destroy Mezner, but upon doing so, Hayden becomes infected with the virus himself, changing his life forever. The infection grants him new powers, such as turning invisible, creating shields, and using a special glaive that combines melee and ranged combat capabilities.
Hayden can also use regular firearms like shotguns or pistols, which can be taken from enemies or purchased on the Black Market, and modified to suit his needs. He must fight his way through hordes of enemies to survive.
The game features a multiplayer mode with two different options: Infection, where one player becomes a fully mutated Hayden and the others must hunt him down as ordinary soldiers, and Epidemic, where two teams compete to kill each other's mutants in a battle for survival.