Infector of Souls is one of the superuniques guarding the seals in the
Chaos Sanctuary, and arguably the most dangerous one.
The easiest way to kill him (or any other CS superunique, but you can use this trick only on one of them per game) is to break the seal spawning him as the last one (after breaking the other seals and killing the other superuniques). Then you just need to kill the remaining superunique only; his minions will die right away after that.
Teebling Admin