Demonic Form

You call upon the strength of the Nine Hells to infuse you with demonic power. After casting this spell for one minute, you can choose to either reap Demonic Form's benefits instantly, or you can choose to wait. If you choose to delay the effects, you can complete the spell as an action at any time, as long as it is within 12 hours of casting the original spell. You are not concentrating on the spell during this time, and the duration does not go down until you complete the spell. 

Upon completing the spell, you transform into a demonic version of yourself. You grow to one size larger, grow wings which grant you a flying speed equal to your walking speed (if you do not already have wings), and your skin hardens, granting you 4 AC and 6d12 temporary hit points.

Additionally you grow a set of long claws making your unarmed strikes now deal 18 (4d8 + 2) slashing damage, and allowing you to replace your attack roll modifier with a +6 for unarmed strikes. However, while transformed you are not allowed to cast any spells except for cantrips, and you may not benefit from beneficial spells cast on you by another creature.

Finally, you gain advantage on Constitution checks made to maintain concentration, if you do not already have advantage.

If this spell is cast using a 9th level spell slot, you gain an additional 3d12 temporary hit points, and Demonic Form no longer has a maximum duration. You do not need to concentrate on the spell while taking a short or long rest, but while in this form you require twice as much food and water, and you have disadvantage on any checks that require you to complete delicate tasks (such as writing, lock picking, or holding fragile objects). If these activities would not normally require an ability check, they now require an ability check (DC determined by your DM) without disadvantage.