eternal flames

You chose a fire of any size within 10ft of you and then you think of a single target. the fire you chose then turns black and starts spreading in all directions (spreads 25ft a turn on flammable surfaces and only 10ft a turn on none flammable surfaces) this fire does 2d8 damage a turn to anything it’s spread to and the fire will keep spreading until it has destroyed the target you chose. the fire can only be put out by a wish spell, waiting 1000 days or the castor of the spell dying and just to balance this spell the fire can also damage the castor but does only half the damage.