The Soulbringer (BlackSkullOfMastery) is an ancient artifact dropped by the Lich boss specifically for players of the Necromancer class. It serves as both an artifact and a path to mastery for Necromancers.
It's an ancient artifact, crafted by first necromancers to collect souls of the fallen one's. Sensing your necromantic abilities it warded itself with a powerful spell. But since the spell is that of the old times, it's primitive structure is fairly easy to break.
When used, the Soulbringer offers the choice between:
If choosing to become a Lich:
The Soulbringer mechanics are implemented in BlackSkullOfMastery.java, which extends the MasteryItem.java class.