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 alive and 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.
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 ever living does help his chances) That would be completely awful if it weren't for that fact that Szeras is a Harbinger of Destruction and so carries a Str 8, AP 2 weapon, which combined with the fact that he costs slightly more than a baseline overlord makes him just meh, even though his whole unit gets defensive grenades.
The main reason people get him
Szeras's big saving point is that he has a mechanical spider instead of legs which makes him arguably the coolest looking model in the Necron army.