Wait in the same room as Mordred in the Ethereal Void and wait for him to trigger a conversation with you. The time taken for this to trigger can be about 30 mins.
Note: One precaution you can take in the Ethereal Void is to pick up the vials which you can find scattered on the floor - specifically there's one which pops in the room Mordred is sitting in. It casts 'teleport' so you can drink it upon noticing someone in the Void for a quick escape.
Mordred the damned expresses his discontent with a loud BLAH. 'My life, such as it is, or is not, shall never return to what it was before I summoned that hideous creature.' Mordred the damned says to you. 'Daelyn, i ask of you, go to my killer and gain for me some means of revenge, or payback.'
After the conversation, you have a limited time (TBC) to kill The Mighty Balrog, Azathoth.
Note: It is unclear if the person who spoke to Mordred has to deal the killing blow in order to obtain the mark.
For sending Azathoth back to whatever void he was raised from, the Powers have granted thee a Mark. You have been granted The Mark of Payback! You gain 47 quest points! You gain 20 practices!