Witches by the Dozen

If this WQ bugs for you, due to the Blighted Horrors not becoming attackable, there is a trick that seems to prevent this from happening every time. Simply put, don't aggro and AOE all the [npc=134310] at once. Having them all near-simultaneously break their channel seems to be what bugs the horror from attacking. If you, instead, separate even one, and then follow with the rest, the spell should break properly, releasing the Horror and making it attackable. After several bugged mobs, using this method saw the channel stop properly every time for me.

Must complete the Break on Through quest chain which is part of Drust Do It. before you can do this one. You won't be able to enter the area until you have completed Break On Through which will then open up more quests for that area. You pick up quest Barrier Buster from [npc=132193] at the Corlain FP area which is the prelude to the Break On Through. These two are the ones required to open the gates to this area where the WQ is located. There are more quests in that Break on Through chain. Please see others comments under the achievement.

Once you have completed Break On Through more quests will become available. You can do all the new quests while the WQ is up; they're in that area. The quests are also part of Storming the Manor part of this achievement. The final quest of this chain will give you Waycrest Manor: The Fallen Mother a dungeon quest.
[npc=131638] will have To the Other Side
[npc=131638] will have A Traitor's Death, Hexecutioner, Ruinous Rituals.
There are a couple of side quests near that area and one inside the area on your left once you enter the gate you just opened. I couldn't remember enough of their details. Some you get from an NPC by a rock near the Waycrest Manor/North Corlain area.

Still bugged, to the point now the witches don't even channel. The only horror actually working is the one at the main gate.