The Pain is the first boss fought in Metal Gear Solid 3: Snake Eater.
The Pain belongs to the Cobra Unit, an elite group of soldiers commanded by The Boss. Snake encounters him during Operation Snake Eater in the Soviet wilderness. The battle takes place in a cave and emphasizes the game’s unusual boss-design tradition, including swarms of hornets controlled through The Pain’s special abilities.
The Cobra Unit members are identified by emotional or physical concepts rather than ordinary personal names. Their titles include The Fear, The End, The Fury, and The Sorrow. These encounters form a sequence of major tests for Naked Snake as he advances toward his mission’s political and personal climax.
The Pain is sometimes confused with The Boss because both are members of the same military circle. The Boss is the mission’s principal target and commander of the Cobra Unit, not its first boss encounter.