Orikan the Diviner
Orikan the Diviner is one of the most powerful Crypteks in the entire Necron Empire. In particular, he is an astromancer, specializing in reading the future from the patterns and motions of the stars.
He is acknowledged as the very best in his field -- apparently, he was able to predict the Fall of the Eldar, Horus Heresy, and the coming of the Tyranids sixty million years in advance -- which gives him a massive ego. He has no respect for the nobles who hire him, but his reputation renders him effectively immune from punishment, because any Overlord who wishes to harm him is paralyzed by the thought that Orikan must have foreseen that sequence of events and taken steps to prevent it or take pre-emptive vengeance.
In reality, Orikan is not quite as good a forecaster as the Necron nobles think he is. He can predict events in more detail than any other astromancer, but even he makes mistakes. He makes up for this with his mastery of time travel -- if his prediction is erroneous, he goes back in time and manipulates events to conform to the prediction (in spite of the unforseen events that this can bring about later). Why he doesn't just go back in time to tell himself what will happen, a far easier activity than determining what's going to happen from the most subtle manipulations of the universe is another mystery of stupid writing in 40k. On the other hand, any Back to the Future fan worth his salt would probably tell you that it's most likely because doing so would create a HUGE temporal paradox, and the universe is messed up enough as it is.
Orikan's big plan is to somehow harness the power of the alignment of the stars in the indeterminate future. He has a chance of doing this in miniature during games of Warhammer 40,000 with a special rule called "The Stars Are Right" (feel free to make Cthulhu jokes as desired). He gains a hefty stat boost if you roll equal to or under the current turn number (and he no longer reverts if this roll succeeds again). For maximum nostalgia, go full Mumm-Ra and invoke the "ancient spirits of evil" whenever you roll that die (the rule actually includes evil laughter -- "bwahahaha!").
There is strong evidence to suggest Orikan is a C'tan who managed to avoid the Poké Ball prison: his complete disregard for the necron nobility, the fact he's actively avoiding close scrutiny, and the small tidbit about his empowered stats being identical to that of a C'tan Shard. His equipment helps align him even further, giving him their trademark 4++ and AP2. However, that would also mean that he's one of the weakest C'tan in existence since at his most powerful he's only as strong as a small piece of the rest of the other C'tan. Maybe he is a freed shard or something, but minor shards only have the intellect on par with a simple animals and masquerading as a brilliant scientists is way above their capabilities. Or maybe Orikan Empowered is just the Necron equivalent of Daemon Princedom. Or maybe he's really Dio.