Illuminor Szeras
"Anyway, that's how I lost my medical license."
Illuminor Szeras is a Cryptek lord who, with help from the C'tan, developed the process for biotransference. He is obsessed with unlocking the mysteries of life because he thinks he will eventually become a god, and wants to make sure he knows what he's doing once that happens. 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.
Szeras vivisects living creatures while they are awake and without painkillers to find out how they work. 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. 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. He is arguably the most evil of Necron lords, 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 loveable kleptomaniac rivalling the Blood Ravens, Szeras is essentially Josef Mengele in a robot body, or maybe space Nagash because he is a evil skeleton that tried to achieve godhood, but not aiming at chaos and has not succeed yet.
On the table
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.
At 110 points he is a decent army buff and tank/TEQ killer. He probably shouldn't be your Warlord, but he works.
In 8th Edition
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 loses 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.
The main reason people get him
Szeras's big saving point is that he has spider-legs which makes him arguably the coolest looking model in the Necron army.