The Magma-Wyrm

Ecology
The Magma-Wyrm is a massive, bio-engineered serpentine beast created by the shamans of Jarl Throm-Ka. It is composed of segments of flexible blackened iron fused with living magma and elemental fire. It is used as a living siege engine, capable of tunneling through the thickest glaciers and melting the foundations of dwarven fortresses in seconds.
Combat Mechanics
The Wyrm leaves a trail of molten rock wherever it moves, creating difficult terrain that deals fire damage to anyone standing in it. It can burrow through ice and stone with ease and possesses a "Magma-Breath" attack that deals massive fire damage in a 60-foot line. It is entirely immune to fire damage and regains hit points when it starts its turn submerged in lava or magma.
Loot
The scales of a Magma-Wyrm are made of "Volcanic Iron." These scales can be harvested and used by a master smith to forge armor that provides permanent immunity to fire damage and resistance to cold damage.