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:

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).

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:

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.

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

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.

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.

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.

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.

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 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.