Demonic Polymorph

Chose a target if not willing must make a wisdom saving throw (can fail if wanted) The target must succeed on a Wisdom saving throw or shape-shift into Demon form for the duration. That form can be and fiend plus Tarrasque or Kraken The target’s game statistics are replaced by the stat block of the chosen Demon, but the target retains its alignment, personality, creature type, Hit Points, and Hit Point Dice. To is cast at the end of three turns and has to concentrate the full time. If concentration is broken, must take an action to reactivate. 

The target gains a number of Temporary Hit Points equal to the Hit Points of the Demon form. The spell ends early on the target if it has no Temporary Hit Points left.

The target can't cast spells while in demon form, other than its spells

The target’s gear melds into the new form. The creature can’t use or otherwise benefit from any of that equipment.

Can only be cast again when your body has healed (Up to your DM). Can be preformed as a ritual (takes 10 minutes). When effect ends, you can not cast any more spells till long rest.