Illuminor Szeras
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.
Szeras vivisects living creatures while they are painfully alive and not-so well to find out how they work. He especially likes to use Eldar in his experiments. If the screaming gets too loud he just turns his hearing down.
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.
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.