There are some cool concepts here, but it's a little rough around the edges.
A level 40 soldier should have AC 56 and other defenses 52 on average; this guy's defenses are way too low.
Looks like his hit points should be 712: (8 * 41 (level +1) = 328) + (con score = 30) = 356 * 2 (elite) = 712. Given the immortal assistant, I'd reduce this to about 650- still enough to be challenging, but not basically twice as many as should be.
Some of his powers are confusing.
Geaskai blightsight seems like it could be simplified, and several of its elements are really kind of whack- range 175 automatic hit?? I think that's a bit much. I would rewrite it like this if it were me:
Range 50*; automatic hit; the target is dominated until the end of tis next turn and is immune to this power until the end of the encounter.
*This keeps a "Whoa, that's really long range!" feel to it without being totally over the top.
Immortal Assistant is cool but needs to be clarified. Again, this is how I'd do it:
Immediate Interrupt* (when an attack hits the exiled prince and he is bloodied; at will): The exiled prince's immortal assistant appears and suffers the effect of the attack in his stead. The assistant then vanishes. The assistant has 300 hit points; once these are gone, it is destroyed, reforming after the exiled prince has a short rest.
*You had it set up as a reaction.
Checkmate: I would eliminate the Bluff check and simply create 20 illusionary doubles. I'd also add a minor action teleport "Switch places with a duplicate" ability.
Birthright looks like a recipe for slowing down play while you look at the pcs' items for half an hour. I would dispense with it entirely, personally.
Zero Requiem is confusing and doesn't say whether it's a melee, ranged, etc. Also, it dominates but it attacks AC? I'm not quite certain of your intentions for it- how does this attack look?- so it's hard to give suggestions, but it needs clarification if nothing else.
Radiant Judgment is, again, not marked as melee, ranged, etc. Also, is it supposed to be doing radiant damage? The damage on it is very low, especially for a 40th level dude.
Identify Discovered seems like a silly thing- basically, 1 in 4 attacks are wastes of time? For a 40th level guy?? I'd completely remove this.
Power of the Kings- I'd move this to the "Immune" line of the stat block and eliminate the text, it just clutters the stat block up.
Also, I'd recommend reordering things- your basic attack ought to be first, and the stat block should be ordered in a way where the dm can easily see how the monster should play. Immortal Assistant should be near the bottom, most attacks near the top, etc.