Magmin

Long ago, a tribe of human or Morlock demon-worshippers in Sub-Terra was transformed into humanoid creatures of molten rock now known as Magmin. They dwell in the deepest depths of Sub-Terra, near natural pockets of magma and lava vents, since they prefer the super-heated environment. Surface drilling and blasting, along with natural eruptions, have occasionally driven Magmin to the surface, where they attack anyone not of their own kind before retreating down below the surface once more.
Magmin are stocky, wide creatures with skin like cooling magma: a dark rocky coating over seething hot liquid rock. Their eyes and mouths glow with heat. They’re capable of grinding speech, but more often simply grunt or growl. Their terrible heat makes even touching them dangerous. Magmin are immune to heat and can wade unharmed through pools of lava. Indeed, they’re quite uncomfortable in cooler environments.
Some Magmin are still demon-worshippers, led by a magic-using priest, while others seem entirely agnostic. Whatever they once were, they appear entirely asexual now, reproducing rarely by infusing life force into pools of magma, from which arise new Magmin, fully-grown and able to take on roles in their society in fairly short order.