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Lightless Arbor Dungeon – The Mountain Beneath Boss Guide

Last Updated: February 15th 2024

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Lightless Arbor is a dungeon themed around darkness whose primary function is to serve as a gold sink. It features a light based mechanic that buffs players and debuffs enemies, valuable rewards through a 'juicing' mechanic where spending gold increases rewards, and a boss fight with two very different stages.

In this guide we'll cover layout, boss mechanics, and the rewards you can obtain from the Dungeon.

If you're new to Dungeons, we also recommend checking out our Dungeon Guide.

The Mountain Beneath

Accessing the Dungeon

To get into the Dungeon you need a Lightless Arbor Key. They drop randomly from enemies but can also be target farmed from Timeline Bosses and the Arena of Champions (Arena not recommended due to low drop rate).

Once you've obtained a Key (or Keys), you need to head to the Lightless Arbor in the Ruined Era. Start at The Surface waypoint and head to the entrance to the north that leads to The Shrouded Ridge. Upon arriving in The Shrouded Ridge head directly north until you reach the entrance to the Lightless Arbor. Right clicking on your Lightless Arbor Key will highlight this section of the map.

For more information on farming Keys from Timeline Bosses, check out our Monolith Advanced Strategies Guide.
For more information on farming Keys from Arena of Champions, check out our Arena of Champions Guide.

Dungeon Rewards

There are many valuable rewards in the Lightless Arbor including Items you acquire through The Vaults of Uncertain Fate, boss specific Uniques and Dungeon modifier rewards.

The Vaults of Uncertain Fate

The most notable reward is the ability to spend gold on modifiers that increase the rewards found behind The Vaults of Uncertain Fate. Upon killing The Mountain Beneath and entering the next area you will a door that leads to the Vaults. Interacting with this door starts a series of choices. You can choose to add a modifier to the vault for gold or skip it. Every additional choice beyond the first increases the cost of adding additional modifiers, eventually reaching the point where you either reach your modifier limit or every modifier is too expensive.

Rewards from the Vaults of Uncertain Fate. Each pile of gold has a treasure chest underneath it.

Once you're done choosing modifiers you can open the vault. All of the modifiers you selected will be accounted for and the reward will be inside one or more chests. If enough modifiers were selected this will result in an explosion of items!


There are 4 different Uniques that The Mountain Beneath can drop, with different tier requirements for each:

  • Peak of the Mountain drops from tier 1 and higher.
  • Foot of the Mountain drops from tier 2 and higher.
  • Face of the Mountain drops from tier 3 and higher.
  • Core of the Mountain only drops from tier 4.

Dungeon Modifier Rewards

When running a Dungeon you stack modifiers that have both a positive and a negative effect. There are two sources of modifiers:

  • Any tier of Dungeon beyond tier 1 has a modifier attached to it. This modifier changes daily. Consider saving some keys for a particularly good modifier that you can farm.
  • Every door you open to move to the next part of the Dungeon adds a modifier. There are two doors in total, one to go from floor 1 to floor 2, and one from floor 2 to The Mountain Beneath.

Lightless Arbor features an additional set of modifiers for The Vaults of Uncertain Fate. When interacting with the vaults players choose these modifiers by paying gold.

This dialogue box appears when you place a Lightless Arbor Key in the dungeon door, allowing you to choose your difficulty tier, and see the modifiers.

Dungeon Modifiers

There are many modifiers that the player can encounter from increased chance to find a certain type of Exalted item, to increased crafting materials. Higher tier Dungeons have better versions of the lower tier modifiers as well as new ones (See Difficulty for more info on tiers).

Door Modifiers

Tier 1 Reward Modifiers

Tier 2 Reward Modifiers

Tier 3 Reward Modifiers

Tier 4 Reward Modifiers

Vault Modifiers

These directly affect the rewards that will be found in The Vaults of Uncertain Fate. Tier 1 has lower quality choices than tiers 2-4 largely due to not being able to drop Exalted items. Because of area level the tier 4 dungeon is the only one that can drop Exalted items with a tier 7 affix. Additionally, the maximum number of modifiers that can be purchased increases with tier level:

  • Tier 1: 8 modifiers
  • Tier 2: 9 modifiers
  • Tier 3: 10 modifiers
  • Tier 4: 11 modifiers

Tier 1 Reward Modifiers

Tier 2 Reward Modifiers

Tier 3 Reward Modifiers

Tier 4 Reward Modifiers

Dungeon Ability

Upon starting the dungeon you have Burning Amber attached to you which illuminates the area around you. Enemies outside of the illumination deal much more damage and take much less damage. You are also able to move the amber to a target location and recall it by activating your ability. This ability lets you target enemies but is also used to burn pyres, which is necessary to progress through the dungeon as well as defeat the boss (more in the Boss section).

You can leave the Burning Amber attached to you, but being hit while it's attached results in the amber losing some of its illumination (the amount you lose depends on how much damage is dealt) . The illumination can be restored by slaying an Amber Elemental.


One of the map layouts. The roadblocks are the brackets that stretch across some of the paths.

The Lightless Arbor has several map layouts, all with a somewhat oval shape. On top of the layouts all Dungeons have a system of pseudo randomized roadblocks. These change every Dungeon you run, and their purpose is to block some paths while leaving others open.

Each run consists of two different maps with one for each floor, followed by the boss which is always in the same arena.


While the normal enemies in Lightless Arbor are not especially dangerous compared to most enemies in the game, the large amounts of Poison Damage can cause issues for builds susceptible to DoT damage and there are a few enemies with mechanics that need to be avoided.

The Rotweaver has the ability to Poison players from a distance with the ability Weave, which has no cooldown. A single Rotweaver isn't threatening, but in a group they can take players down quickly. Avoid accumulating too many stacks by staying on the move so they can't easily target you.


Difficulty scales through Dungeon Tiers and Modifiers. There's also an added layer of difficulty through the 'roguelike' aspect of Dungeons.

Dungeon Tiers

Every Dungeon has 4 tiers that you can choose from once they're unlocked. Tier 1 is available by default, and once you complete the Dungeon it unlocks tier 2, etc (Tier 2 of this dungeon will also unlock at level 65 if it hasn't been already). Higher tiers have higher level enemies while also adding stronger modifiers. You also need to complete higher tier Dungeons to get certain Uniques from The Mountain Beneath as well as add more maximum modifiers to the Vaults of Uncertain Fate (see Dungeon Rewards for more info).


Selecting any Dungeon above tier one grants your first modifier. This modifier offers a certain reward, but also increases the difficulty of the Dungeon by adding Increased Damage and Increased Health to all of the enemies (including the boss). You also need to use two doors to progress through the Dungeon (one to go from level one to level 2, and one to go from level 2 to the boss room). Each door also adds a modifier to your run from that point forward, making subsequent rooms more difficult, but also more rewarding, than the previous room.

Lower tier Dungeons have small modifiers, while higher tier Dungeons get substantially more difficult. For example, the strongest modifier at tier 1 adds 30% Increased Health and damage to enemies, while the strongest tier modifier at tier 4 adds 180%. When selecting your modifiers in a Dungeon, don't just pay attention to the rewards it offers, make sure you can also handle the difficulty scaling.

A door leading to the next floor from a Tier 1 Dungeon
A door leading to the next floor from a Tier 4 Dungeon

You Only Get One Shot...

There are no extra attempts in Dungeons. If you die at any point in the Dungeon, you lose all of your progress and need to use a new Key to try again. You need to balance the risks of starting over with the rewards offered to you in order to maximize your Dungeon experience.

If you see this, you've lost your key and you'll have to start the entire Dungeon over again

Pre Boss Strategy

Whether you want to clear a map or rush straight to the boss depends on the modifiers you pick up. For example, if you get a modifier that increases the drop rate of Exalted Amulets, it may be worth your time to clear the map. Keep in mind that higher tier Dungeons offer higher drop rates on modifiers. This means that full clearing lower dungeon tiers is unlikely to be rewarding and you're better off pushing to higher tiers and farming there.

If the modifiers aren't enticing enough to farm the Dungeon (and they often won't be) then there is no additional benefit of killing enemies in a Dungeon. Your best strategy, if you can survive it, is to just rush the doors and get to the boss as quickly as possible. Every floor has two doors so if the first door has a modifier that is not desirable or too difficult, you can look for the second door and see if it's better.

Boss - The Mountain Beneath

The Mountain Beneath is a fight with several one shot/near one shot attacks as well as two phases (The Mountain Beneath and Stone Titan's Heart). You will not be able to defeat this boss without knowing how his mechanics work.


Pyres on the left and right of the arena must be set on fire by using Burning Amber. The Pyres are blocked by a Root Wall that must be destroyed in order to access them. Once they finish burning The Mountain Beneath will take damage and upon burning both he will start phase 2.

Boss Strategy

There are two phases to the fight. The first involves fighting The Mountain Beneath on a platform. He's invulnerable so the real goal is to burn both Pyres with Burning Amber. Once both Pyres have burned The Mountain Beneath will fall over and his mouth becomes a doorway to the second phase where you need to kill Stone Titan's Heart. Here are some strategies for each phase of the fight:

The Mountain Beneath

  • At the start of the fight immediately head towards the right Pyre on the far right side of the arena. Starting on the right side will make it easier to dodge the first round of Shockwave. You need to destroy the Root Wall that is blocking the Pyre first. Once destroyed, place your Burning Amber on the Pyre and head towards the center of the arena.
  • Try to avoid Rockfall and kill any Titanspawn and Mountain's Roots while waiting for the first Shockwave. Once The Mountain Beneath uses Shockwave he attacks on the left side first, so stay center-right to easily avoid it. As soon as the left side attack is over immediately move to that side of the arena to avoid the right Shockwave.
  • As soon as the right Pyre is lit recall your Burning Amber and head to the left Pyre. Take out the Root Wall and place your Burning Amber. Now head back to the center and repeat step two. If you're fast enough in destroying the Pyres there won't be a second round of Shockwave but if not you'll need to deal with it one more time
  • As soon as both Pyres are lit The Mountain Beneath falls over and you can enter the second phase through his mouth.

Stone Titan's Heart

  • Unlike The Mountain Beneath, Stone Titan's Heart is attackable and needs to be killed to end the fight so deal as much damage as you can while avoiding his more threatening mechanics
  • Titanflesh in this phase aren't very threatening but should be dealt with when possible to avoid being overwhelmed from multiple damage sources.
  • Spore Beam is the most dangerous mechanic in this phase. As soon as you see the beam head to the opposite side. The beam won't cover the entire arena but on higher phases it will cover most of it, so make sure your positioning is solid.
  • Energy Wave does a lot of Poison Damage. You can either cleanse the Poison or avoid the mechanic altogether. Be aware that sometimes the safe zone for Spore Beam and the damaging area of Energy Wave can overlap and the only way to avoid both is a very well timed movement ability or exceptional move speed (and some luck) making cleanse the more reliable option.
  • All of the other abilities are either very well telegraphed and easy to avoid or deal low enough damage to not be especially threatening on their own. Avoiding Spore Beam and Energy Wave are the most important parts of this phase.

The shear number of mechanics in this fight and the distinct differences between the phases can be overwhelming for new players, but in reality there are only a few mechanics that players need to learn in order to reliably defeat this boss. Understanding how the Pyres work and then avoiding Shockwave, Spore Beam and Energy Wave are the keys to your success.


Coming soon!


  • The Vaults of Uncertain Fate is a valuable gold sink that can only be accessed through this dungeon
  • Burning Amber is a dungeon mechanic whose usage is required in order to complete the dungeon and defeat the boss.
  • Enemies are relatively easy, but be on the lookout for Rotweavers, Rotting Hollows, Lightless Hive & Gloom Stingers, Hollow Spriggans, and Amber Elementals.
  • Dying in the dungeon causes the player to fail, lose all progress, and lose the Lightless Arbor Key they used!
  • The Mountain Beneath is a two phased fight with several mechanics that you need to know in order to defeat him. Know how to destroy Pyres as well as how to avoid Shockwave, Spore Beam and Energy Wave.


Written by McFluffin.
Reviewed by Facefoot, Lizard_IRL, Raxxanterax.

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