"Anyway, that's how I lost my medical license."
Illuminor Szeras is a Cryptek who, with help from the C'tan, developed the process for biotransference. He is obsessed with unlocking the mysteries of life because he
thinks knows he could and will eventually become a god, and wants to make sure he knows what he's doing once that happens. The universe’s first megalomaniac with a god complex who wants to be a competent god. Only in 40k There are other examples, but they just ask nicely. Dissecting creatures at the molecular level has also given him insight into improving the Necrons' robotic bodies, either with more powerful servos or thicker armor.
Both hated and envied by the other Necrontyr, the Necron Cryptek (a plasmancer to be precise, though he's dabbling in biomancy) known as Illuminor Szeras is one of the most reviled beings in the whole galaxy. The C'tan might have provided the required knowledge to the Necrontyr for the biotransference of their consciousness into their new bodies of living metal, but it was Szeras who made it a reality. Even then, he saw it as the first of several steps on the path to ultimate evolution, a journey that would end as a creature not of flesh or metal, but as a god of pure energy. Until that day, Szeras is driven to take full advantage of his android form. After all, no longer must he sleep nor deal with the thousand frailties and distractions to which flesh is heir. Essentially he's the necron equivalent of a transhumanist, one who sees technology as the means to that end (with a god complex thrown in). Of course part of this may be due to the fact that he stumbled upon some...very sensitive data regarding his “patrons” namely that a human was apparently given the divinity he craves.
Szeras vivisects living creatures while they are awake and without painkillers to find out how they work, all so that he can know what he's doing when he reaches godhood. He has been on the brink of reaching the desired understanding for many Terran centuries, yet somehow final comprehension - understanding the nature of the soul - always escapes him. He's considered the possibility he will never fully understand either because it's beyond logic or that one has to be among the living to fully understand, but he refuses to accept this. Given his role in roboticizing the Necrontyr, he's an expert at modifying and enhancing their bodies. On occasion, Szeras' talents are in such demand amongst the Necron dynasties that he can name his own price -- invariably a harvesting raid targeted against a world of his choosing. Since he considers battles a waste of time and resources at best, the only reason Szeras ever goes into battle is to make sure his employer doesn't renege on the bargain.
Since he goes through a lot of captives, when he's not in his laboratories he's accompanying Necron armies, exchanging his ability to augment the Necron troops for captives to experiment on. He especially likes to use Eldar in his experiments, because they give more interesting results to him. The subjects are kept in stasis so they don't die as they're cut to pieces. He watches each one to make sure he doesn't miss anything and take notes. If the screaming gets too loud for his liking, he just turns his hearing down (not to avoid feeling empathy - he considers them inferior beings unworthy of consideration, and to him this is a sort of mercy- but because he personally doesn't like the noise).
Also, it turns out that he is the cause of the Pariah Nexus formation during the Psychic Awakening and he is working for a mysterious new Patron by augmenting their troops. At the same time he's still trying to become a godlike being of pure energy much like a C’tan and is closer to his goal than ever before.
He is arguably the most evil of Necrons, as he just doesn't give a fuck about anything except his own personal ambition and research. Whereas Imotekh wants to conquer the galaxy because he thinks he's the best person to lead it, and Trazyn is a lovable kleptomaniac rivalling the Blood Ravens, Szeras is essentially Josef Mengele in a robot body, or maybe space Nagash because he is an evil skeleton with a penchant for deadly unethical experiments that tried to achieve godhood, but not aiming at chaos and has not succeed yet. In 40k he could simply be like Fabulous Billy, or a robot version of the Dark Eldar Haemonculi, given his penchant for modifying his own robot body and those of his people.
On the Tabletop
Szeras's main selling point is his special rule Mechanical Augmentation. At the start of the game chose one unit of Immortals or Warriors to get +1 strength, toughness, or BS for the whole game. the failings of this ability are that you can only use it on Immortals or Warriors and that you have to roll for which bonus you get. It's nothing to scoff at, mind, as a 15-20 group of warriors with 5 S/T/BS is a scary thing.
Szeras has the same stats as an Immortal but with an extra wound and four attacks, which he may or may not get to use because of his very Necron-ish initiative 2. (Although he does have a Sv3+ RP4+ W2 statline to keep him alive) That would be completely awful if it weren't for that fact that Szeras carries a S8 AP2 Lance weapon, which combined with the fact that he costs slightly more than a baseline overlord makes him an alright choice. He also does give the +1 RP to both his own unit and any unit in 6" of him, which in a Decurion means that all those units still have a 4+ RP against ID. Terra
At 110 points he is a decent army buff and tank/TEQ killer. He probably shouldn't be your Warlord, but he works.
Probably the best HQ for Necrons. His Mechanical Augmentation can be used any number of times (once per turn), though not on the same unit twice. The bonus from it is now random on a roll of a D3. Also he gives +1 to Reanimation Protocols of friendly Necrons within 3". Basically he cuts your losses in half, since now Necrons come back on 4+. Couple that with a Resurrection Orb and watch your opponent cry as he cannot inflict any real casualties to your units.
As of Psychic Awakening 9: Pariah, our many leggy skelly has gotten a new model, and HOOOOOOO boy does it look cool. He is now a lot larger and more ornate looking. He has basically replaced the much-loved pariah.
This big model has also given Szeras a serious power boost as his eldritch lance is now Assault 1d3 while also being AP-3 in melee. His legs also add 2 extra AP-2 attacks in melee and make him M8. And he'll want that because killing units in melee lets him make another augmentation out of turn. And to top it all off, he can make enemy psykers trigger perils on any doubles.
The New Big Reason to Get Him
Szeras's big saving point is that he has spider-legs which makes him arguably "one of the best Necron models ever" (GW's words, which, for once, we wholly agree with), and because of his Psychic Awakening rules he can now counter Psykers, thus saving the lives of countless Canoptek Scarabs, which were previously charged with the singular duty of taking Smite directly to the face.