Actually, these guys are the placeholders for "named" lava mephits and pumice collectors that spawn randomly in the Xuzl miniboss area. Nothing special about them - they behave similar to the standard issue inferno and flame mephits that populate the caves. Ie, pretty much a straight melee fight with a fire-based 250 direct-damage bolt-style (obeys line of sight, travels outward from the caster in a line to the target, is not insta-damage like the "shock of" line) spell called "Ball of Lava".
After killing many of these the other night, I have learned quite a few things.
They are mages, yellow to 65 (so 66 or 67), and have around 25k hit points. They randomly drop ornate molds/patterns (hats, gaunts, boots, bracers, arms), and spectral / ethereal parchments.