The original idea would be that he was an animated sculpture infused with life by one of the Winter Court Queens, sent out with the mission to track down mortals that failed to uphold their end of a bargain with the court. You know the usual failure to deliver their first born, punishing people who insult the court, and the like.
He'd be sent into the mortal world to locate and with the warlock build he'd use ample applications of Charm, Devil's Whispers, and the Geas invocations to force them into upholding their end of the bargin. The Humanoid shape invocation would allow them to move through the world unhindered, as well as granting them all movement modes when needed. The Vecna Blooded template would mean that the target couldn't predict or divine that he was coming or even uncover his disguise before the Fey Repo Man wanted him to.
The hexblade paladin build is an interesting option as well. Little more violent than I had in mind but could be just as fun.