Snap Caster

You are able to call on the power of the Weave at a moment's notice, but at a cost. Once per round, you can cast a spell with a casting time of 1 action or 1 bonus action as a reaction. After casting the spell, you must make a Constitution Saving Throw (DC 13 + the spell's level). If the spell requires material components and you don't have an Arcane Focus you have disadvantage on this saving throw, but otherwise you don't require material components to cast the spell. On a failure, take 1d10 + the spell's level Force damage. On a success, take Force damage equal to the spell's level. This damage can not be mitigated by any means. If this damage drops you to 0 HP, make one Death Saving Throw immediately.