This is the ghost of a dragon, which you fight in ghost form as the final boss fight.
This creature's stat block appears to be a modified version of the Ghost's stat block. The Ghost is a CR 4 creature. Ebondeath has also been given a CR of 4, but it has:
significantly more hit points (225 compared to a Ghost's 45), AC of 15 as opposed to 11, a way higher save DC of 20 vs. Possession (rather than the Ghost's 13), and Legendary Resistance. Everything else appears to be the same, including the attack modifier and damage of the Withering Touch attack, and the save DC of Horrifying Visage (13 for both stat blocks).
This looks like a mistake to me, and given that some traits have been updated where others have not been, it also looks like a lazy effort without paying much attention to the details.
Ebondeath is clearly stronger than a Ghost, and therefore surely must be of a higher CR that 4. What CR should1 this stat block have been given? As a bonus question, what else should be updated to make things more consistent with the CR it should have been given (meaning the Withering Touch attack, and should the save DC of Horrifying Visage be 20 as well)?
1. Note that by "should", I mean if it were to be derived from its stat block, not just an opinion (the word "should" often has that association, so I just wanted to make that clear).