To possess dark magic and control tainted spirits, you must obtain devils' energies.
Get a Masked Control Runestone from Masked Goblin Smasher in Nombardi Town at the Twin Lion Mountain. Then, get energies from the Flame of Hatred and the Blaze of Greed at the South Lion Mountain.
These dark energies will improve your skills.
When you return, head to Schilling Village in Prism Island Prairie and look for Sage Babama.
Only he can fuse the darkness with your soul and make you a Demonologist.
Child, I believe you are ready to become a dark Demonologist.
The road ahead will be difficult, but I have faith that you will prevail.