Marrow Pass is a dungeon on the Earth level of Remnant: From The Ashes. Like all dungeons in the game, its exact location will be random, depending on your own dynamically generated world.
The Marrow Pass is home to the Cult of the Root, and there are two main threats you can deal with here to get XP and items.
The Root Nexus
The Root Nexus is a demonic tree that you will find in an antechamber on the west side of the map. It has a pretty stacked health bar, so make sure you have plenty of ammo. When you shoot it, it will begin to summon in enemies and will do so until it dies.
It is a good idea to cycle between damaging the tree and clearing enough enemies to stay safe. When the tree is dead, any enemies it has spawned in will automatically despawn. You will get 250 XP for killing the Root Nexus.
The Root Cultist
In the other antechamber, you will find the Root Cultist. This is a human NPC that will attack you when you approach the room. He is not particularly tough, and you will get the Braided Thorns ring from his body when you kill him, and 250 XP.
Navigating the dungeon
To get through the dungeon, just keep to the left. This will bring you around in a big circle, and when you get all the way through you will be able to open a metal gate, making a shortcut to the exit.
The dungeon is filled with lesser enemies, including some of the exploding type, so be cautious. There are also a few tougher enemies, but you will hear the audible warning when they are close to you.