Some members of the group had tried and failed to clear the Treasury before so we knew it would be challenging but we were lucky enough to have a well balanced group with Guardian, Minstrel, Lore Master, Hunter, Burglar and Champion (Throg). Throg (level 55) and the minstrel (53) were both below the 56ish level of the instance but the others were all higher so it more or less balanced out.
It is a well designed enjoyable instance set in a circular chamber with balcony around. As you enter, a boss absconds to a locked side chamber with his treasure leaving the fellowship to clear trash ringed around the balcony. Once the trash are cleared you have access to a puzzle which must be solved in order to open the locked door. Clearing the (including six mini bosses) also gets you drops for several side quests but there isn't enough to go around so we had to do a second run to get all the drops we needed.
The puzzle involves rotating six statues in the correct order in order to illuminate a central pendulum which opens the door. Each statue is turned using a wheel on its balcony mounted pedestal but turning any wheel out of order resets all the statues. The order seems to randomly change on every run of the instance so a bit of trial and error is required to get it right.
In a chest beside each wheel you get either a yellow or blue crystal shard. These are needed in the following boss encounter so make sure they are spread around the fellowship.
The first boss waits in his side chamber. He has two main abilities that must be overcome. Every 30 seconds or so he changes from corporeal form to ethereal form in which he has huge damage mitigation (90% I think). This change can be delayed by using one of the yellow crystals which will extend the timer for his corporeal form. Each crystal is on 5 minute cool down so spread them around the fellowship. The boss also has a series of nasty debuffs that get applied to random members of the fellowship. These debuffs are announced by an eye over your character's head so you can avoid them by getting out of line of sight but they can also be cured. The most serious debuff seems to be a wound which does 500 damage every 2 seconds so it is vital to avoid or cure this. Our LM was able to cure it and we didn't have to run out of line of sight but other have reported that this can be a killer. We never got to use the blue crystal shards - these say that they extend ethereal form but we could see no reason why you would want to do that.
Exiting the side chamber after killing the first boss you run into twin Troll bosses. This is a tough encounter. Not only do these guys hit hard but when they get close together they get even stronger. Keeping them apart is very difficult because they seem to share aggro and damage in some fashion that we couldn't quite work out. As if that wasn't enough they have some kind of progressive debuff that gradually lowers every member of the fellowships maximum health (at one stage Throg's maximum helath had dropped from over 4000 to less than 200!). We tried rooting one and tanking the other away but it didn't work. We tried using two tanks to hold the trolls on opposite sides of the circle but the shared damage made it almost impossible to control aggro. The one useful thing we discovered was that the progressive debuff goes away in the little side chamber so we pulled the trolls in there but of course it became even harder to separate them in this little room.
We died many times (dropping out for repairs at one point) but eventually we killed them by kiting them around the little room and hitting the running trolls as best we could. Since they do melee damage this kiting works but it was a very slow process because its is very hard to land hits on a running troll.
Our method though effective was very inelegant. I am sure we are missing something. Perhaps with precisely balanced tanking we could have kept them apart but I have no idea how you would manage this. I am also curious about the blue crystal shard which we never got to use. Could they do something for this encounter?
After killing bosses we got to loot their chests. Mainly legendary weapon upgrades but there were a few pieces of armour. Throg got a cloak with +might, + morale that will suit him nicely at level 56.