If you were following the event speech and clues, you find that Casca was abducted from the Royal Council and dragged through the British Sewers through Minoc and eventually ended up in the Ethereal Fortress.
Since lot of people don't know the Ethereal Fortress and the hidden dungeon there here is how to get there..
First take Moongate to Ilsh Spirituality. There you will run North to the Spider Cave. (yellow dot starting point, green line rough path to take, red dot entrance to spider cave)
The exit from the cave is to the East.
Run East and you will see a circle of pillars, step on the runic symbol in the middle to teleport to Reg Volom (Ethereal island and Fortress)
Run North into the fortress and you will see a black hole leading down, there are lot of ghouls and spectres so be ready.
Now run North and you will see a scroll on the ground at one of the intersections. The scroll has the password you need to say to enter next level. So stand near the scroll and type "masteropenthydoor".
Go through a door and when you get to an intersection go West.
Now you are at the first magically locked door. Here is where it gets REALLY dangerous as you will see ancient liches in that room (even paragons for more fun). How you get through is up to you, most people resign to just getting killed and have someone rez them in the room with Casca since you can say the magic words as a ghost. Stealthing is rough since you will be revealed when speak the words, but could time it right to say the words and hide right away before getting targeted. An easy way is if you have honor worked, then the hard way of having all the passwords on macros and running as fast as you can.
Anyway you choose to do it, stand in front of the door and say "Painless Operation" then head to north door for second password.
Next password is "Safety In Numbers"
Careful up the stairs as next door can be physically opened and balrons and or demons could be waiting on you up the stairs. Open door and run to north door of that room with the balrons and big yew tree (in an under ground dungeon that one heck of a tree).
Password is "futile"
Ok, again be careful (assuming not running through as a ghost)
as again up the stairs could have ogre lords or trogs waiting on you. Go into the room and head to the east door.
Password is "concerto"
Head east and meet Casca. Remember if nothing else you have to say "help" to him to get the scroll for the reward.
Told you it was a lot of pictures..
Due to the high likelihood of dieing, I suggest either make sure going with someone that can rez you, use honor or use sacrifice to self rez.
