The ebony clock in 'The Masque of the Red Death' represent Death.
Explanation:
Ebony is a dark wood. The ebony clock represents death, the inevitable, the unsurpassable, the mightiest of the mighty death. Just like no one can stop death from wielding its slaying wand upon the helpless mortals, none can stop the ebony clock from chiming or its pendulum from swinging. The sounds that it produces disheartens and discourages the mortals who can do nothing but bear its brutalities.
All the major symbols in ‘Seven Ages Of Man’, ‘Annabel Lee’ and ‘Dream Within A Dream’ are similar to the ebony clock as they all underline the idea that man is not all-powerful but a puppet in the hands of the more powerful, a pawn in the scheme of things who is at mercy, living till he is fated to live and leaving when the mightier forces will him to.