Naerandir has told you he knows the story of Othrongroth, the Great Barrow, but that the reality of the place as it exists now must be masked beneath the splendour of legend, lest cautious fear grows into terror.
'You have reforged the key! This step was necessary to complete your story and your foray into this terrible place. Now, you must return to the deepest parts of the Great Barrow and defeat Sambrog, the ancient wight-lord who makes the Barrow his home.'
Sambrog is at the furthest end of the Great Barrow, in the deepest halls.
Naerandir has asked you to defeat the ancient wight-lord Sambrog.
Naerandir is awaiting further word of your deeds outside of the Great Barrow.
You should return to Naerandir and tell him that Sambrog is slain.