Vecna's Wicked Absorbtion V3

You take a feature of a creature you've (most likely) defeated it can be a limb (ex: bone demon tail) or a power also if you're trying to use this to replicate something like a beholders eye beam then you must take the beholders extra eyes first to. If strain does not work for two reasons (Rather, your level exceeds the monsters CR, or you are level 20) make a con save equal to their CR.

Strain the harder it is to cast a spell the more it hurts your mind for How strong the being you expect to use this spell so it deals damage using this 1d10 x Monters CR (Or More CR = more dice) -the players level if this is higher than your HP you instead pass out and don't cast it and are left at 1 hp(bypass this for level 20 characters because you can't absorb 21 CR or higher).

Dark Magic This spells connection to Vecna, the arch lich can corrupt its caster. The DM will decide (in secret) how many times it takes. (ex: if you use it and other magic a lot it may be set high like 50 but if you are very minimal with you magic it will be set low like 10-15) Once the limit is reached the DM can give you one random flaw. (Use a flaw you can get from sleeping in an evil plane for 1 day as an example) change Lawful to chaotic and good to evil. Evil characters ignore this.