So I have the Dust of Restful Death, but I do not have the markings on my map of where the tomb is. Also, I got a letter a while back to meet someone who works for the underking. Here is the problem. when I got that letter I was in the middle of doing 2 other quests and by the time I was done the person had let. I do not know what to do next. No one has any of the main quests for me to do, I just completed the lich's quest with the scarb soul and I gave that back to the King of Worms.

Any idea's on what to do?

The location of Lysandus' tomb is given as a reward for completing that quest for the Underking. It's in Menevia if that helps you in any way, but I fear you're screwed.

You can also get the location of Lysandus' tomb by doing quests in Menevia. It is on the list of dungeons they send you to.