Skull Expert.

You gain proficiency in "Medicine, History, and Religion"

When you make a check using "Medicine, History, or Religion" that involves skulls, you can add double your proficiency bonus to the check, instead of your normal proficiency bonus.

Once per day, you can cast Speak with Dead or Create Greater Undead without expending a spell slot. These spells require only a skull to cast instead of their normal cost, which is consumed upon use.