Hedge Event Lair of Terris Thule Access (Plane of Nightmare)
In order to gain access to the second half of Plane of Nightmare (Lair of Terris Thule) you'll need to complete either the PoN Hedge event OR you'll need to complete the Alternative Quest to bypass having to complete the PoN Hedge event. For more information on the Alternative quest follow my previous link.
The Hedge even itself is pretty straight forward, to start you need to speak with Adroha Jezith in the Plane of Tranquility. You can find her in one of the buildings that surrounds the zone line to Plane of Nightmare and the Plane of Health.
Hail Adroha Jezith
/say Tortured by Nightmares?
This will grant you a Character Flag which is all we need from these NPCs. After speaking with them zone into Plane of Nightmare and head northeast to Thelin Poxbourne. He's across the river inside of a little hedge enclosure. Hail him and follow his dialogue through.
For the next part of the quest you'll need to complete the PoN Hedge Maze. A total of 18 characters can enter into this quest at once but you shouldn't need more than a single group to complete it. All you need to do is follow Thelin Poxbourne as he travels through the Plane of Nightmare maze and protect him.
It takes about 15 - 30 minutes to complete and Thelin walks extremely slow. He stops in 7 or 8 different places and each place spawns about 3 or 4 enemies which you'll need to kill. The map above is a picture of the entire maze that Thelin walks through. He goes around the maze counter clockwise at first but then switches to clockwise after picking up the first one or two pieces of the dagger.
Once Thelin reaches the end of the entire maze (which is the central most portion of the maze with the water fountain) you'll fight the a construct of nightmares which upon defeating him will drop a Dagger Blade Shard. Return the Dagger Blade Shard to Thelin and that will complete the Plane of Nightmare Hedge quest.
Don't forget to hail Thelin Poxbourne before leaving the PoN Maze!!