Cloak of Corruption comes from Shadowlords.
The Seer at Serpent's Hold gives the quest for Cloak of Death. You need one Cloak of Corruption and 10 Threads of Fate.
The Seer at Empath Abbey (North of Yew, West of Yew moongate) gives the quest for the Cloak of Life. You need one Cloak of Corruption and 10 Threads of Life.
The Seer at Lycaeum gives the quest for Cloak of Power. You need one Cloak of Corruption and 10 Threads of Thought.
Threads of Fate are automatically given when killing corrupted souls in Magincia, but not a rate of 100%.
Threads of Life are given by restless souls at Magincia when you use Spiritspeak near them.
Threads of Thought are given by forgiven souls at Magincia when you use the Exorcism spell on them. The northern part of Magincia doesn't allow to use the Exorcism spell, stay in the southern and center area.