Although it sounds ethereal and high-minded, the spirit mastery is going to be familiar to anyone that's played as a necromancer in any role-playing game. Instead of encouraging your enemies to seek salvation, you're going to be blasting them with life-stealing plagues, lower their resistance to damage with skills like necrosis, and force them to flee for their lives with visions of death. In addition, your powers over the undead will allow you to spread the joy to your teammates, allowing everyone near you to increase the damage they deal to undead enemies and reduce the damage that they take in return.