He's back baby! My initial thoughts (I didn't dig into CR math or anything yet).
There is a lot to like about this bad boy:
1) Overall this Dragon really feels like an epic creature. Its got nigh immunity to magic (with those saves your passing even the strongest level 20 saves (DC 19 by default) on anything but a 1, and of course you get advantage. You have a solid AC (with shield especially) and hitpoints. You have restraining, stunning, and incapacitation to keep the party tripped up. You have teleportation when the party releases their battlefield control spells, and of course the ultimate breath that counters almost any magical buff or gimmick the party can throw at you.
Offense wise, a killer breath weapon that does enough damage that an immediate death is possible for even a 20th level character (and of course a power word kill as a backup).
I think this guy has all the tools to take on 20th level parties and have them quaking in their boots, I like it!
2) Considering an at-will shield spell, I think its worth also noting this in the reaction section, as its a major power boost to the Dragon's defenses (AC 26 with that on....yeah!)
3) Scorching Ray is such a feeble attack that its barely worth mentioning in the statblock. Even with resistance to necrotic the necro blast is normally better, as it should be for an undead dragon. I would just remove the scorching blast, I think its overkill.
4) Withering Grasp, the text is a little confusing here. I think the intention is you loose necro resistance for an hour...and can make a save every round to get out of the restrained....but people could also read this as a save every round to get back their neco resistance, so could just use a little clean up.
5) Quicken Breath. I like the mechanic. I do agree with the poster above that on a quick read its very easy to think it works with either breath, so you may want to clarify it further. Something like "when using your breath weapon (Necrotic Blast version), or something to that end.