As a quick sneak peek, I want the boss (a necromancer wizard) to have a homebrew cantrip, and I really like this. Ghostly Leach:
Make a ranged spell attack against the target. On a hit, the target takes 1d4 necrotic damage. Gain temporary hit points equal to the damage dealt by this spell. This spell’s damage increases by 1d4 when you reach 5th level (2d4), 11th level (3d4), and 17th level (4d4).