As per Allak's:

First you must obtain the key to Jaled Dar's Shade's lair. Seek out Zlandicar in the depths of Necropolis. He has the key you'll need and you can get it one of two ways:

1) The 'easy' way: Kill him and loot it.

2) Work your Zlandicar faction up and kill Neb (leader of the Paebla clan in Necropolis). Bring Neb's head to Zlandicar in exchange for the key to Jaled Dar's Shade's lair.

Jaled Dar's Shade yields the key to Sleeper's Tomb. You'll need to give him either a Talisman from any of the five dragons of the first brood (Zlandicar, Klandicar, Yelinak, Sontalak or Lendiniara)..

OR a "Shard of Hsagra's Talisman" which is dropped uncommonly from top giants in Kael Drakkel (King Tormax, Vindi, Statue).

After handing one of these items over, Jaled Dar's Shade will reward you with your key.