Find a portal with ghosts streaming out of it and click it.
Your vision will go purple and somewhere near by will be a crystalline rock.
Click the rock to destroy the portal, then slay any beings from the other side that passed through the veil.
Note that there is a bug with the portal next to the tree near the entrance - no rock appears
Remove a final anchor and kill the Unspeakable Horror- This is a ^ version scaled to your level and is a hard fight - if it keeps being too hard, you can go mentor yourself down 5 levels, go back in, and unmentor... everything will then be 5 levels lower and easier to kill, including the name. Try not kill the horror's adds as these heal the main mob on their death. After killing, you then have to click the portal and the stone one more time to update.