The Skull Goblet can be used as a spellcasting focus for Warlock spells. As long as the goblet is filled with the blood of a humanoid creature, you can cast Blood-Writ Bargain, targeting that creature regardless of whether it is willing. The blood then evaporates from the goblet in trail of black smoke and the goblet cannot be filled again before the spell ends.
Optionally: You gain an expertise die with spell attacks against humanoid creatures that are charmed by you.