Scarab was with Mevolent during the War, and was one of his top assassins. He was framed by the Sanctuary for the death of Esryn Vanguard and was arrested. He survived two centuries and was released at the start of the forth book as an old man, wanting revenge against the Sanctuary, and Guild in particular.
Scarab later kidnaps Guild's family to force Guild to blow up 82,000 people at Croke Park Stadium. The plan fails and Scarab is then arrested. Presumably, he will die in jail. It is unknown as to where he is being held.
Dreylan Scarab is categorized as the perfect genius: he is highly calculative, extremely intelligent and utterly, completely ruthless. He is also exceptionally wrathful and will turn on anyone who dares wrong him. He shows, in his climactic speech to Thurid Guild, that he believes the Sanctuaries were not needed - pompous, self-important individuals such as Guild were hated and loathed by people like Scarab. Scarab was extremely unpredictable, as his next move was never completely clear to his opponents.
It is presumed that Scarab is an Adept, but it is not confirmed. He seems to know a lot about symbols and which symbols cancel out which. It is entirely possible that Scarab is a Necromancer, given how he managed to bring Spacegrace back as a Zombie.