Tunnels of the Forsaken - dungeon

After you finish Althea's Ashes, go directly south of Piken Square to that pit area. Ghost of Althea should be right outside the dungeon and give you the next quest "Elegy for Those Left Behind".

Then in Piken Square, get "The Cost of Survival" from Kopp the Quick.

Then Ghost of Althea hands you the final quest on the first level of the dungeon.

Doombox just released a video if you need a visual guide. There is also this forum post.

Also need to be in Reforged Mode.