The thermonuclear smoke devil is the boss variant of the smoke devil. It can be found in the room next door, where the normal smoke devils are. It will count as a boss kill in your slayer log if you kill these monsters.  The nuclear smoke devil is the superior variant of the regular smoke devil. It has a one in 200 chance of spawning when you are killing smoke devils on a slayer task. When you kill it, you have a chance of getting superior monsters' slayer loot. These can be the eternal gem and the imbued heart.