I’ve posted an update of some of the changes I’m making to my Helltamer compendium class here: http://vindexus.net/2017/03/28/corrupting-the-helltamer-binding-world-update-5/
I’m going to try out a system where the Helltamer tracks “corruption”. The spells that they get to cast scale with corruption, so more corruption = more powerful spells. However they’ll sometimes have to fight the powers of hell, and a higher corruption makes it harder and more dangerous.
http://vindexus.net/2017/03/28/corrupting-the-helltamer-binding-world-update-5/