The Mountain Beneath is a formidable boss encounter located within the Lightless Arbor dungeon in Last Epoch. Known for its massive stone-and-plant titan form, this fight is a major challenge and key to accessing valuable endgame content, including the Vault of Uncertain Fate Last Epoch Gold. The Mountain Beneath fight is notable for its multi-phase design, intense mechanics, and the necessity of managing a unique light-based resource system called Burning Amber.

The fight begins on a large rectangular arena where the Mountain Beneath is initially invulnerable. Players need to strategically light two Pyres located at opposite ends of the arena to progress. Each Pyre is protected by a Root Wall that blocks direct access and must be destroyed first. This is accomplished by maneuvering the Burning Amber — a glowing, light-emitting entity bound to the player — near the Roots and Pyres. Carefully managing this light is crucial since the dungeon is enveloped in darkness, increasing damage dealt to players outside the Amber’s radius.

Once the Root Walls are destroyed, players place the Burning Amber on the Pyres to ignite them. This must be done while avoiding the boss’s devastating Shockwave attacks and dodging falling Rockfall markers on the ground. Shockwaves hit the left side of the arena first, followed by the right, requiring swift repositioning to avoid one-shot damage. Killing summoned Titanspawn and clearing Mountain’s Roots during this time helps reduce pressure, but the priority is always protecting the Amber and igniting both Pyres quickly. If done efficiently, the boss’s second Shockwave phase can be skipped.

After successfully lighting both Pyres, the Mountain Beneath collapses, signaling the transition into the second phase. Players enter the Mountain's mouth, shrinking the battlefield and facing new threats, including destroyable mines and a deadly green laser attack shooting in two directions, requiring precise positioning. The second phase requires high awareness and the ability to manage adds and environmental hazards while maintaining DPS output on the boss to end the encounter.

This fight rewards players with the key to the Vault of Uncertain Fate, a mysterious structure containing powerful modifiers that influence the dungeon’s loot in exchange for risk. The Vault offers exalted items and unique rewards valuable for progressing endgame gear. Accessing this Vault is only possible after defeating the Mountain Beneath, making mastery of the fight both rewarding and essential.

To succeed, players must balance aggression with survival, leveraging their Burning Amber effectively and adapting to the boss’s attack patterns. The fight’s complexity and difficulty make it one of the toughest challenges in Last Epoch, demanding skillful evasion, timing, and resource management. Mastery of both phases and efficient Pyre ignition are key to overcoming the Mountain Beneath and unlocking the Vault’s riches buy Last Epoch Gold.