There are 3 Crux quests. In speaking to a bard in my guild, he stated that all three get you an item that ultimately combines to either summon a big baddy or gives some kinda item.
The 3 Crux quests are:
The Crux of Cairnesworth Hall
The Crux of Castle Landsview
The Crux of Trengal Keep
The Crux of Trengal Keep was completed by a group in my guild last weekend. The mod that you have to kill at the end of the dungeon run (I suggest you be on Legend of Trengal Keep part 6 for this since you have to crawl the same dungeon to get to the mob) is a 6 dot 30. In order to spawn him, though, you need to get a hold of Nishek's whip which is a rare drop off any mob in there. I have read on Curse Gaming (
www.curse-gaming.com) that there is a quest giver in the dungeon who tells you to go find the whip.
Once you kill Nishek, you will get Nishek's Nosering.
The Crux of Cairnesworth Hall has a reward of the Houndstooth Torque (I am pretty sure thats what it is called).
I would assume that once you have all 4 items, one would be able to do the follow up quest.