Demonic Spirit

You touch a creature. it must succeed on a charisma saving throw or become cursed for the duration of the spell. if it is willing, it instantly fails the saving trow. a creature may retake the saving throw at dawn every day, except every failure increases the dc by 1.

the curse has the following effects on the target:

the targets alignment changes to chaotic evil and becomes an NPC under the dm's control.

its mental ability scores become equal to one randomly determined demon's mental ability scores.

its melee attacks deal an additional 4d8 fire damage on a hit.  

it gains immunity to fire damage.

its spells that would do fire damage instead deal that much + 2d10 instead.

at any time, it can force any creature of its choice within 5 feet of it must make a constitution saving throw or take 3d6 fire damage.

at any time 24 hours after the casting if the curse is active, the creature may roll a d20. on a 20, the creature dies and in its place,than the demon of which the target had took the mental abilities of appears in its place. the demon then attacks anything within its sight. the demon disappears after 24 hours, returning to its home plane. on a 10 or higher, nothing happens. on a 9 or lower, the creature may make a charisma saving throw of the normal dc - 5.

when cast from a 9th level spell slot, you may choose which demon the target gains the mental ability scores of.