Voadam
Legend
If I'd have been designing Sithicus I'd have fought hard for the new darklord to be a shadow or reflection of Soth from one of his magic mirrors, adding an extra layer of unreliability to his own memories as he fought against the suspicion that he wasn't real and the recollections he was cursed with weren't even his. And maybe one night a month under Nuitari, Sithicus could be whole again, the forests verdant and sun-dappled, the elven cities bustling and elegant, as Lord Soth, Knight of the Rose, rode forth with his companions ... but it's all a collective hallucination or memory, spawned from Soth's broken mirrors, as he runs through his resentments again in his mind, trying to ensure that This Time everyone else (who were responsible for what went wrong, of course...), get it right and things work out as he'd planned.
That is a fantastic idea. A ghost of a death knight. Alternatively even if he is not the Darklord, he could be in the castle.