The Magma Behemoth (click for map image) spawns in the mines (PH "a worn slag golem").
For your "arginhiz.txt" map file (note for all Argin-Hiz named map points, see this quest entry):
The Magma Behemoth, itself, is reportedly mezzable. It casts "Golem Thrashing" (50' Hatelist AE for 50,000 dmg) + Lavapunch (random single-target 1-second stun + 35,000 dmg + 5,000/tick DOT) + Magma Bubble (self-buff, +300 AC, +2,500 to Fire/Cold Resists, 50% chance to reflect spells).