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.
To collect both the greater souls and the lesser souls you must click on the soul object that drops when you kill the monster.
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 - Den of evil (5% fire resist)
Griswold - Tristram (500 defense)
Act 2
Fangskin - Claw Viper Temple level 2 (5% lightning resist)
The Summoner - Arcane Sanctuary (5% faster cast rate)
CHALLENGE LEVEL 30 - Mechanized Destroyer - Stony Tomb level 2 (5% chance of staggering blow)
Act 3
Sszark the Burning - Spider Cavern (5% fire resistance)
Ismail Vilehand - Travincal (5% Increased Attack Speed)
Act 4
Izual - Plains of Despair (+50 to life)
Infector of Souls - The Chaos Sanctuary (10% faster hit recovery)
Act 5
Shenk the Overseer - The Bloody Foothills (5% summoned minion damage and life)
Lister the Tormentor - Throne of Destruction (5% magic resistance)
Normal Resumé
You should finish normal with 12 souls. (10 from lesser souls, 1 from challenge lvl 30, 1 from Greater soul)
Nightmare
Act 1
Rakinishu - Stony Fields (5% run speed)
The Countess - The Forgotten Tower Level 5 (5% poison resistance)
Act 2
Radament - Sewers level 3 (5% poison resistance)
Coldworm the Burrower - The Maggot Lair level 3 (5% cold resistance)
Act 3
Geleb Flamefinger - Travincal (10% elemental damage)
Bremm Sparkfist - Durance of Hate level 3 (5% lightning resistance)
CHALLENGE LEVEL 60 - The Swamp King - Swampy Pit level 3 (5% all resistances)
Act 4
Hephasto the Armorer - River of Flame (500 defense)
Lord De Seis - The Chaos Sanctuary (10% physical damage)
Act 5
Eldritch the Rectifier - Frigid Highlands (5% run speed)
Frozenstein - Frozen River (5% cold resistance)
Nightmare Resumé
You should finish nightmare with 25 souls. (10 from lesser souls, 1 from challenge lvl 60, 2 from Greater soul and you start with 12 souls from normal)
You will most likely not finish challenge level 60 before hell, so 25 souls should be your goal here. Don't forget to come back for this soul!
Hell
Act 1
Blood Raven - Burial Grounds (5% minion damage and life)
The Smith - The Barracks (500 defense)
CHALLENGE LEVEL 99 - Stitches - Plaguelands (5% increased attack speed, +50 to life)
Act 2
Blood Witch the Wild - Halls of the Dead level 3 (10% physical damage)
Fire Eye - Pallace Cellar level 3 (5% run speed)
Act 3
Toorc Icefist - Travincal (10% elemental damage)
Wyand Voidbringer - Durance of Hate level 3 (+50 to mana)
Act 4
Grand Visier of Chaos - Chaos Sanctuary (5% magic resistance)
Act 5
Pindleskin - Nihlathak's Temple (5% run speed)
Nihlathak - Halls of Vaught (10% minion damage)
Bartuc the Bloody - Throne of Destruction (5% chance to bleed)
CHALLENGE LEVEL 80 - Spawn of Ifrit - Icy Cellars (5% missile speed)
Challenges Recap
Challenge souls :
Act 2 Normal, Stony Tomb, Rocky Waste, max level 30. Time limit: 3 minutes (5% chance of staggering blow)
Act 3 Nightmare, Swampy Pits, Flayer jungle, max level 60. Time limit: 4 minutes (5% all resistances)
Act 5 Hell, Icy Cellar, Ancients Way, max level 80. Time limit: 2 minutes 20 seconds (5% missile speed)
Act 1 Hell, Plaguelands, Burial Ground, max level 99. Time limit: 9 minutes (5% increased attack speed, +50 to life)
Greater Souls
Act bosses each drop their greater soul, which is a socketable soul that you can put inside your Annihilus.
Normal :
Mephisto [1]
Nightmare :
Andariel
Baal
Hell :
Duriel [2]
Diablo
Rift Bosses :
Trang'Oul, located in Hell Act 1, Cathedral, min lvl 75.
Ctoggha, located in Hell Act3, Disused Reliquary, Kurast Causeway, min lvl 75.
List of Greater Souls and their bonuses can be found in this page: Uniques/Greater Souls
Riftstones
Tier 1 Riftstones
Crazed Archer - Catacombs Riftstone (5% increased attack speed)
Vengeful blacksmith - Sewer Riftstone (10% physical damage)
Tier 2 Riftstones
Legacy's legend - Cave Riftstone (10% elemental damage)
Mechanized destroyer - Crypt Riftstone (5% bleed chance)
Tier 3 Riftstones
Fallen angel - Durance of hate Riftstone (5% maximum cold resistance)
Deathspeaker - Tomb Riftstone (5% maximum magic resistance, 5% maximum poison resistance)
Tier 4 Riftstones
Swampking - Maggot layer Riftstone (5% faster cast rate)
Guts - Dungeon Riftstone (5% life regeneration, 1500 defence)
Tier 5 Riftstones
Nemesis - Temple Riftstone (5% maximum fire resistance, 10% increased attack speed)
Guardian of terra - Worldstone Riftstone (5% maximum Lightning resistance, 10% faster cast rate)
Conclusion
There is actually 51 souls to collect, which makes a total of 52 souls with your soul on a completed annihilus.
