Souls
Certain super unique monsters in certain difficulties are able to drop their soul on death. There are two tiers of soul drops, lesser souls and greater souls.
Contents
Lesser Souls
Lesser souls give you small passive bonuses on your annihilus. Typically there are 2 per act per difficulty, usually dropped from super unique monsters related to a quest. There are also other lesser souls available from challenges and other custom content.
Normal
Act 1
Corpsefire Griswold
Act 2
Fangskin The Summoner Mechanized Destroyer
Act 3
Sszark the Burning Ismail Vilehand
Act 4
Izual Infector of Souls
Act 5
Shenk the Overseer Lister the Tormentor
Nightmare
Act 1
Rakinishu The Countess
Act 2
Radament Coldworm the Burrower
Act 3
Geleb Flamefinger Bremm Sparkfist The Swamp King
Act 4
Hephasto the Armorer Lord De Seis
Act 5
Eldritch the Rectifier Frozenstein
Hell
Act 1
Blood Raven The Smith
Act 2
Blood Witch the Wild Fire Eye
Act 3
Toorc Icefist Wyand Voidbringer
Act 4
Grand Visier of Chaos
Act 5
Pindleskin Nihlathak Bartuc the Bloody Spawn of Ifrit
Greater Souls
Act bosses each drop their greater soul, which is a socketable soul that you can put inside your annihilus. Mephisto's soul drops in normal, Andariel and Baal in nightmare, and Durel and Diablo in hell.