Welcome to endgame: Korthos Isalnd, heyton's rest.

First things first, what you'll need to tackle this quest.

1. Be ON DESTINY. This is not the kind of quest to level off destinies in. Ideally, you will have six members of level 26-28, with maxxed destines.
2. You absolutely NEED good crowd control. This is not optional. A way to keep the hordes of undead busy while your group focuses on killing the sahaguin by the altars.
3. A dedicated healer will be needed, especially if the party can't bring down the sahuagin before the orange named undead - they hit like a truck.
4. A trapper is absolutely required if you want the bonus chest.

You will enter the quest and see a man standing by a lever. This is Jacoby Drexelhand, and he will plague you throughout this storyline. Ignore him for now. Mass death ward, pro elements, freedom of movement, standard buffs that you've been using throughout your ddo career. Now, talk to Jacoby Drexelhand, who will open the door into the most epic chain DDO, and flee. And a good thing, too, as you'll find out later, he would probably wipe the floor with you at this stage of your career.

At the fork, head left. Locate the secret door if you can (DC is around 95). Break the breakables, then hope to god the trapper can find the acid trap. If he can't, don't worry about the chest and head back down the hallway that was on your right. If you loot the chest, go back and do the same after collecting your min lvl 28 new gear.
Proceed with caution throughout the hallway. The human cultists are quite strong, though not much stronger than EE gianthold mobs. Kill the casters first, followed by the melee. The archers certainly hurt, but they attack slower. At the next fork, head right. Here is the first of three major fights before the endfight. Take advantage of what is probably the only reason this quest is completeable: The smaller undead are unintelligent, so halt undead allows no save. Quickly, everyone must destroy the altar. Use haste boosts. Ideally, you'll have two furyshotters in the party: HAVE ONE and ONLY ONE unload here. With luck, you will destory the altar before the insanely powerful undead rises. Now focus on the sahuagin - be careful of it's spellcasting ability, help the healer out watching your own health.

Head straight south after this room, and handle the room the same way - the second furyshotter should unload on this altar. Now, make your way straight, you can use the shrine on your left if needed. If you can wait, that would be best, save your sp for the endfight. On your right you can see you still need one more crest. Ahead, again on you left, you will see the optional chest - but if you couldn't find the door, you probably can't find the trap. Someone with improved evasion for half damage, energy sheath for half damage, acid absorption 33%, resist acid AND protection from acid, combined with 900+ hp MAY survive, but it isn't recommended.

Back in the main hallway you will find the last door. The first furyshotter should be off timer - have him unload on the last altar, standard CC on the undead.
Shrine, and rebuff. The good news is that the end fight is not much more difficult than the others. The second furyshotter should unload on the altar. DO NOT BREAK COFFINS, they can spawn the ridiculous bags of HP and damage that are the undead. Halt them and gang up on the sahuagin. She is very powerful, but your group can probably powerheal through her ridiculous attacks - luckily she does not have AoE damage. Once she is dead GRAB YOUR CHEST AND GET OUT while the undead are halted.