This is what one gets when they infuse an elemental with the negative energies found in all undead. It is a humanoid zombie, but it is constantly burning and smouldering, making the bite and physical attacks all that much deadlier.

These particular undead often appear as blackened corpses, who have clearly been through an inferno. The skin is split in numerous places where the interior of the creature is visible. Instead of showing cooked meat and organs, there is a hellish reddish-yellow light, and it appears as if the material exposed through the cracks is still burning.

Which, case in point, is exactly what is happening, although the monsters are not being consumed by the infernal heat. They do not seem to be bothered by the superheated material inhabiting their remains, and despite the temperatures, the remains are not consumed by the thermal dynamics.

These creatures are slow moving and are typically dispatched from a distance with little to no effort, and that is the best way to deal with them.  Trying to engage them up close and personal is a very dangerous proposition, unless the attacker has protection from thermal energy.

Of course these creatures are immune to any sort of thermal-based attack, and shockingly enough, they are also immune to cold-based attacks. It should be noted once they have been dispatched, the bodies will rapidly cool and crumble into a pile of black and grey ash in 1d4 rounds after death.

These monsters are typically found in areas where there is an abundance of heat, such as open crevices which spew lava, active volcanoes and of course on the Elemental plane of fire.

Generally it is easy to spot when these creatures are coming, as the heat generated by their bodies tend to start a lot of wild-fires, so not only are they deadly in their own right, they can cause significant property damage by simply walking through.

Starfinder Creature Index