Masochism

You have tuned yourself to the horrors of the dead and embraced the suffering only bequeathed upon the damned. Once per turn, when you deal necrotic damage, you lose Hit Points equal to half the amount of necrotic damage dealt, but you gain a extra action & the ability to cast another level 1+ spell that turn if you already have done so before. Additionally, when a creature deals necrotic damage to you, you can turn into any Undead of a CR of 10 or bellow for until the end of combat & heal from any necrotic damage dealt to you while in that form, however, there is always a price to pay for power. When you are hit with radiant damage, you take an additional 1d20 radiant damage, and your movement speed is halved until the end of your next turn, also you are unable to use any radiant spells and take damage from healing spells.