Soul Magic

You can use souls to cast magic. 
Soul absorption. 
After you kill a creature, you may chose to absorb its soul into your soul pool. Creatures with a CR over 20 give 2 souls.
Soul Magic.
You may cast a spell which you haven’t prepared in exchange for souls. A Spell’s soul cost is indicated by it's level. By doing this, you destroy the soul and it will not pass on to another plane.
Soul Healing.
You heal yourself (only yourself) by absorbing souls. 2 souls = 1d6 of health.
Soul Pool.
You can store up to 3 souls at lvl. 3, gaining one extra soul slot per extra level.