Expecto Petroleum

A blazing orange portal opens, and burning black gasoline pours out, covering all creatures of your choice within 60 ft. A creature that you target with the spell must succeed on a Dexterity Saving Throw or take 6d8 fire damage and be covered in gasoline. While covered, a creature’s speed is reduced by 10 ft. At the end of the creature’s next turn, it takes 5d8 fire damage, and takes the damage for every consecutive turn end. Each consecutive time, the damage is reduced by 1d8. When the damage is reduced to 0, the spell ends. A creature can clean off the gasoline before the end of the spell, but ordinary water fails to remove it. A Greater Restoration spell or similar magic removes the gasoline.