Souls can be equipped to your commanders to increase their stats.
Each commander has a number of equipment slots equal to the maximum rarity of that commander (e.g. 4 slots for a 4/4 rarity commander or 5 slots for a 4/5 rarity commander). Each slot can be equipped with either gear or a soul, thus it is possible to stack several souls on one commander.
Souls are obtained by either evolving or absolving a max ascendency commander. Evolving a lower rarity commander will yield an (S) soul, whereas absolving a maximum rarity commander will yield an (L) soul. (L) souls are stronger than the previous evolution soul for the same commander.
Souls may also be used to fuse to a commander. When a soul that meets the minimum requirements for a fusion slot is fused to a commander, the corresponding stat of the commander is increased by a fixed percentage. A greater rarity will yield better percentage, and previous souls can be "Overriden". When all four fusion slots for a commander are filled, the commander reaches hero status and the skill charge time is reduced by one turn.