Wait in Mordred's room for a random period of time, can be 2-20 minutes.
Note: this mark is entirely CPK. Be sure to have a form of levitation and a light, but beyond that bring as little as possible. It is very easy to get CPKd in the void, and wind up stuck. If you lose levitation, eat the flower that repops at Mordred's room.
There are various methods of getting to Mordred in the Ethereal Void. Some known ways include entering rune clocktower at midnight, and drinking gold potions that can be obtained from the shopkeeper in the south-west corner of the market square in New Rigel.
The gold potions have a chance of transporting you to either the Faerie Plane or the Ethereal Void. If the potion takes you to the Faerie Plane, you should find a worldgate and gohall in order to save the potions. They only have a chance of getting you to the Ethereal Void if you drink them from Alyria.
To exit the void, simply pick up A nondescript glass vial has been dropped here from Mordred's room, and drink it to teleport to a random location outside of the void. Do not enter the copper door, unless you want to wander the maze and find the down to the hermit. If you do wind up in the maze, locate the down and sayto the hermit "yes" to exit.
Note: If you're doing this mark with other people, be sure you're not formed. Once a player gets the mark, they should leave the room, because their presence will prevent the rest from getting the mark.
For keeping the company of the damned, the Powers have granted thee a Mark. You have been granted the Mark of Gloom! You gain 43 practices!