Prerequisite: Must be able to cast a spell that deals damage.
You have somehow learnt a dangerous forbidden technique - how to bolster your spellcasting using the very essence of your soul. You may choose to deal maximum damage with a spell you cast, instead of rolling.
However, channeling your very soul into raw magic is not without consequence. When you cast a spell this way, you take half the damage the spell would deal. This damage can not be reduced in any way. If a creature dies due to this damage, it's soul is torn, and it can not be restored to life by any means other than a Wish spell.
You can use this ability a number of times equal to your proficiency bonus per long rest.