So... Whenever Bill deals combat damage to a player, assuming they have cards in their hand, they lose the game? Because I get the impression that Bill will be hitting that player every turn after his ability resolves.

Uh yeah; you're never going to let this through unless you cannot prevent it; and one it's gone through once, all your creatures will be as messed up as possible.

It's a slightly interesting attempt at tension - normally one of the things you do to mess up an opponents turn is get rid of the cards in their hand. But that doesn't really matter here, since you'll also be taking their next turn.

Worst, it forces them to concede because it's not even going to kill them off quickly. In multiplayer, for example, you basically get two players resources to cotrol.

Needs massivlely upcosting, a downside, and making mythic. I suggest giving it an 8/8 body as its downside - that way it's usually gonna blow opponents up after stealing only or 2 turns.

Good flavour though. Maybe exile bill when it connects so it only happens once? And probably needs to be more expensive too. But if we have any "control next turn" cards, "on a combat damage trigger" seems a good one to have.