I have done every other quest I can. Except one. I got a letter from an agent of the Underking asking to meet me at a random place in a random building in the Sentinel area. I have no idea where that is. That was almost 3 in-game years ago and he said he'd only be there for one month. I checked the UESP wiki and it said that this quest was required. I need it to find Lysandis' tomb. I have a few questions.

is there anyway I can fix this?

Do I have to make a new character?

Is there some way to reset the quest so I get the letter again?

Do I need to check every building in every town in Sentinel?

I'd rather not skip the quest, but if there's no other way I think I can use the map thing-a-ma-bobber in the games cheat mode. I'm desperate. Please, help.

There is a time limit on the Main Quest (although that might have been removed by one of the community patches).

Sentinel would be in the Alikir Desert.