You must be at least 8th level and have completed the Painting quest for Queen Akorithi to activate this quest. You will receive a letter from one of the Underking's agents which asks for a meeting. The meeting will take place in a tavern in a random town in Sentinel. You will have 30 days from the time you get the letter to get to the meeting, but this is a little buggy. If you travel to any other province besides Sentinel after receiving the letter, the letter will usually disappear from your inventory. The time limit given by the Underking's agent does not jive with what you actually have. Like the Courier quest, go with what's in your log.
Now that you've made buddies with the King of Worms, it's time to foil one of his schemes. He's planted a cursed item in the Blades stronghold (Castle Llugwych in Ykalon province). The Underking wants you to get it before it can do any harm.
There are two ways to get to the item. The hard way and the easy way. The hard way involves getting yourself to the lower levels of the southeast corner of the dungeon and then making a long series of pulling switches and riding elevators. It's a good way to kill a few hours, kill a few critters and work on skills. It gets tiring after a while, so I usually resort to the easy way.
|You enter the castle in a large room with a door across from you and one on either side. Go across, kill the orc and skeletal warrior in the next room. There are rooms to the north and south of you that have treasure piles in them. The northern room also has a warrior-type. Go out through the door in front of you. Follow this corridor around (there's a Mage waiting for you at the bend) until you get to a door on your right (west). Go through it.|
|Go up the stairs to the south (a warrior-type is waiting for you). Follow this corridor until you get to a door on your right (north). Go through this room (a human and an animal occupy the room) and out the west door. Follow the corridor until you get to a 4-way junction with a dead-end (and a warrior-type) in front of you. Turn left (west).|
|Follow the passage through a small room (animal) and up a curving passageway. When you get to the 4-way intersection, go south. Follow this corridor until you get to the door (south). Two critters may come out of the rooms to the east and west.|
|Go through it and south until you get to the T-intersection. Turn east (or cut through the room with the critters and treasure pile). Go up the short ramp and follow the passage until you get to a door on your left (south).|
|Follow the corridor to its end. There will be a door in front of you (east) and a door to your left (north). Through the eastern door is a room with a bad guy. The lever in room 1 should open a door in the corridor above you. The lever in room 2 unlocks an elevator. Now go back to the elevator shaft (3) and take it up (it's already at the top when you get to it).|
|You'll be facing west in a corridor that turns north just in front of you. Take the opening on the right (east). The north-south corridor has another elevator at the north end, by the way. There will be a bad guy (random) in front of the door and a Vampire Ancient behind the door. Behind the door at the end of the short hallway is a room with a pyramid, 2 gargoyles, 1 regular vampire and a critter in the side room. The object you want is on top of the pyramid.|
The Vampire Ancient can be a tough fight for a character 10th level or less. If he's too much for you, let him chase you back to the elevator shaft while you ride the elevator all of the way to the bottom. He will wait for you to come back (no one ever claimed that DF vampires were terribly bright).
From the bottom of the elevator, go east at the first 4-way intersection (there should be an elevator at the end). Ride it up to a Y-shaped passageway and take the right (northern) branch. You'll find a secret door a couple of paces past the corner. Take that elevator up and you'll be at the north end of the corridor leading to your destination (the Vampire Ancient should still be around the corner, guarding the southern elevator shaft).
After retrieving the item, the Underking's agent will remove the curse from the item and let you keep it (it's a random magic item, meaning that your odds of getting something fairly cheesy are pretty good). He'll also tell you where Lysandus is buried and mark it on your map (the tomb is in Menevia province).
To the extent that the quest results in your getting information which you could get elsewhere, I would classify it as an "optional" quest. To the extent that it's a lot easier and more fun than dealing with random assassins, it's a required quest.