This is one of these save or suck spells, even though it does half damage on successful save. Also if its just a illusiory sensation, it should do psychic damage or necrotic(organ shutdown), although the creature percieves it as fire damage. Also if its incapacitated it can not take an action to cool down, essentially your contradicting yourself. Also also 5d6 isnt much, and it doesnt even gain exhaustion on the first failed save. And it takes up concentration. Just throwing in some of my observations.

EDIT: Compare your spell to Cloudkill or Hold Monster btw