The Festering Dark Walkthrough and Map
The Festering Dark is a new Stronghold added as part of Diablo 4 Items the Vessel of Hatred expansion. It's located at the southernmost part of Nahantu, and requires you to use the Reliquary Lantern to light the path through to the boss and remove obstacles as you explore.

If you'd rather explore the eerie caves of The Festering Dark alone, be warned of spoilers ahead.
The Festering Dark Map
When lit with a Reliquary Lantern, each Warding Brazier will activate a Lantern that moves along a set path, illuminating the way and removing a barrier at the end.

On the map below, the colored circles correspond to Warding Braziers, the arrows represent the path the Lantern will take, and the colored lines show which barrier will be removed.
Additionally, there are two extra levels to the stronghold: the Skittering Darkness, and the Vile Darkness. These each have two entrances/exits, one at either end.

The Festering Dark Location
This Stronghold is located toward the south of the map. Approaching from the east, you’ll see the stronghold icon and then a path that ends - we’re heading underground.

The Festering Dark Walkthrough
Head down the cliff and walk inside the mouth of the cave, where you’ll find a necromancer’s corpse and a “lantern radiating with necromantic energy, forming a barrier against the hateful dark.”

Pick up the Reliquary Lantern. You’ll need this to guide you through the Stronghold.
You’ll also see a glowing mark on the floor ahead, the Rune of Trag’Oul. You’ll find these all throughout the Stronghold - interacting with them will resummon the Reliquary Lantern if you’ve misplaced it.

Head north and bring the lantern to the black tendril barrier. The barrier will fall away, allowing you to climb up the cliff.

Continue forward until you see the Warding Brazier. Placing the Lantern in the Brazier will activate it, and the Lantern will travel forward, illuminating the area around it and freeing you up to fight the mobs as you go.

Activate the Brazier and follow the light westward. It’ll encounter another barrier and take it down before dropping on the ground. Pick up the lantern again and head around the corner to the hole taking you down to the Skittering Darkness, where you’ll enter next.

On this floor, follow the curved corridor around, dropping the lantern whenever you need to Diablo 4 Items for sale take down the mobs that have accumulated.