You have long learned that flames can be destructive as well as a sign of renewal and cleansing
When you touch selected target, he gains resistance to fire until the end of the spell effect.
When the target drops to zero hit points, flames shaped like a pheonix burst out of the uncouncious body. Hostile creatures within 25 feet must roll a dexterity saving throw against the caster's spell DC. On a failed save, those targets are knocked prone and take 6d8 fire damage. On a successful save, targets takes half damage and not knocked prone. Allies within 25 feet are surrounded by flames, causing no damage but cauterizing their wounds. They heal for 4d4 hit points.
This effect can only happen once.
When cast at a higher level : Increase all damage die by 1d8 and increase the healing by 1d4 for every higher level used.