At about level 8 in your character's development, you'll receive a message from an agent of the Underking, asking you to meet them in a random town of Sentinel. You will be asked to retrieve an item for the Underking which was stolen by the Blades. The item varies from game to game. (I will attempt to explain how to get to this item at a later date, it is a fairly complex route to explain).

Once you have retrieved the item and returned it to the Underking's agent, the curse will be lifted from it and it will be yours to keep. You will also receive information as to the whereabouts of King Lysandus' tomb. Don't bother going there at this time unless you simply want to build up your character or explore out of curiosity; you will not be able to put the spirit of the slain king to rest until you have obtained the "Dust of Restful Death", sprinkled it on his corpse, and avenged his death.