Raises the souls of fallen corpses. These will surround the Inquisitor for a while before they disappear again. Each fresh soul provides an additional boost to the Inquisitor's attack and defense, as well as resetting the souls' lifespan.
Teleporting out of an area will reset the number of souls you have to 0.
Combat arts that kill large numbers of enemies at once (Clustering Maelstrom) can immediately charge up Soul Reaver to high levels.
Pros and Cons
Pros
With many souls consumed will provide invincibility
...
Cons
All but useless if no souls are around (bosses etc.)
Does not work if the enemies don't provide a corpse after death such as anything that explodes for example: energy beings, demonic eyes, seekers, mutated scarabs and some other types.