This creature appears to be a pool of bubbling and churning molten rock.
Magma Ooze CR 7
The magma ooze is formed of molten rock. Any melee hit deals on the ooze inflicts 2d6 points of fire damage to the attacker. The damage continues for 1d3 rounds, but deals only 1d6 damage during this time.
Environment warm mountains and underground
A magma ooze is encountered primarily on the Material Plane but is thought to have its origins on the Plane of Fire. They are almost always found in or near volcanoes and other warm or hot places. Magma oozes do not approach water and are never found near such sources. Magma oozes can grow to a length of 10 feet, with a thickness of about 6 inches.
Magma Ooze from the Tome of Horrors Complete, Copyright 2011, Necromancer Games, Inc., published and distributed by Frog God Games; Author Scott Greene.