Once per turn, if you kill a creature with a spell, you regain Hit Points equal to twice the Spell Slot level used - thrice if it's a Necromancy spell. Undead and Constructs are unaffected.
Nullify another creature's spell as a reaction. The spell must be 3rd Level or lower. If it is higher, you must succeed a Check to nullify it, the difficulty of which is based on the spell's Level.