He is released by opening the seals in the lower right corner of the level, and must be killed in order to reach Diablo and finish the Terror's End quest. This quest ends the game in Diablo II, and enables a character to advance to Act V in the Expansion.
The Infector of Souls is considered very dangerous for two reasons: his blinding speed and his spawn location. After opening the seal players only have a few seconds to move back to the main room before the Infector spawns; because the seal is often situated around a corner, there usually isn't enough time. The Infector and his minions can swarm players in mere seconds and due to their fire and melee attacks players can suffer quick deaths, even with max Fire Resistance and good defense. For this reason, it may be best to leave the seal that triggers him for last, and then kill the Infector of Souls. While you would not have gained all the Experience you would have if you killed his minions first, killing the bosses after activating all the seals will kill all enemy minions freely.