Head west, then take either of Diablo 4 Items the northwestern paths, as they do converge. At the end of the hall, turn east and head back into the Forgotten Depths.
There are now two different routes you can take, depending on which of the two Braziers you light next. Both of them take you to the boss, but one is a little longer than the other and involves heading through the Vile Darkness. We recommend taking the bottom Brazier, but we'll include both paths in case you want to explore it all.
Bottom Brazier Route
Light the bottom Brazier, and follow it a short distance east until it takes down the barrier.

In this room, there's another Brazier - light it and follow it on a long, winding loop south, north, then southeast, where it'll break another barrier.
The Brazier immediately to your left leads to a dead end, and the one to the northwest takes you back toward the bottom Brazier you lit earlier. Light the Brazier to the northeast, and follow it along to the next room.

This is the final room before the path to the boss, and where the two paths converge.

There are two Barriers and Braziers in this room, and both lead toward the boss. The top one takes you on a more direct route. Feel like fighting some more mobs? Light the Brazier to the south, and it’ll take you on a loop counter-clockwise where you’ll encounter some elites, then break a different barrier to the north, allowing you to access the same path to the boss.
From here, head west and bring the lantern to the "source of the hateful dark".

Top Brazier Route
Light the top Brazier, and follow it east - northeast, where it’ll remove the black barrier. Head south, past the black barrier on your right, and follow the curved path to the east to head down into the Vile Darkness.
Follow the tunnel around, carrying the lantern as you go eastward. You’ll enter a wide chamber with two exits - take either of them as they converge, and continue on until you get to the exit taking you back to the Forgotten Depths.

Head down the stairs to the southwest, defeating the mobs as you go. The west corner is blocked, so you’ll need to head south. If you light the Warding Brazier to the east and follow along behind it, it opens up the path to the northeast, which is back where you started at the entrance to the Stronghold - the north passage takes you back up to the Skittering Darkness.
Instead, go south, and you’ll see a rope hanging across a chasm, which you can traverse to get to the other side. Light the Warding Brazier at the other end, and follow it east, defeating mobs as you go.

It’ll open up the passageway to the south. Pick up the lantern again. Light the Brazier to the east and follow it up where it will remove a barrier to the north and allow you to travel directly west to the boss - if you want to go directly to the boss, that is. As mentioned above, the south takes you on a longer loop, but you'll end up in buy Diablo 4 Items the same place.