The Mountain Sire is a treant boss. At the lowest Threat it has 1350 health, hits for 24 and carries 12 armor; the uber version you meet in the Mortal Arena has ×1.6 the health and ×1.75 the damage. You need Sire's Heartstone to call it out.

It waits at the end of Cragmaw Deep, in place of Imp Overlord, and belongs to Adventure Mode.

Every kill rolls its list 5 times. 8 Uniques sit on it: four ordinary ones splitting 90.0% of it between them, and four Exceptional ones splitting the last 10.0%, which puts any one of those at about 1 in 40 of what it gives up. The Exceptional ones are Bedrock Testament, The Mountain Sire's Spine, Sediment and The Landslide Clause.

It opens every fight with Seismic Slam, then fights with Rockfall, Stone Bulwark, Ground Split, Quarry Charge and Avalanche Roar. The cast bar under its health names each big move as it winds up, so a party can call them out to each other. At 55% health it changes phase: The Mountain Wakes. After each big set piece it stops for a breath: that is the opening to punish. At 15% it makes its Last Stand with Worldbreaker, one named, fully telegraphed finisher, and no burst of damage skips it. The uber version in the Mortal Arena fights in three phases (Quarry, Avalanche and The Living Mountain), grows between them, and adds Landslide and Quarry Collapse. Its weak spot: Hide Behind the Rubble. His charge stops dead in a rubble pile — and leaves him dazed.

It calls in Rubble Roller.