Thran War Machine

Echo and a forced-attack clause are two costs stacked on one body, and the design is honest about the deal it offers. The four-mana entry buys nothing the turn it lands: the construct sits, and on your next upkeep you either pay the echo cost again or sacrifice it. Echo was Urza's-block currency for selling an undercosted creature with a delayed second invoice, and the war machine pays that invoice with a 4/5 frame that would normally cost considerably more. The compelling-attack rider sharpens the bargain. It exists precisely so the discount cannot be cashed passively: you cannot keep it home against a faster clock, cannot hold it back to preserve tempo, cannot decline to commit it. Once it commits, it commits every combat. That turns an efficient body into a liability against decks that want you to attack into a wall or a removal trick, and a genuine threat against opponents who have no profitable answer to a recurring five-toughness attacker. The two restrictions are designed to talk to each other: echo asks whether the body is worth re-buying, and the forced attack answers that the body is only worth re-buying if you intend to be the aggressor. It is a clean piece of late-90s artifact design, a creature priced for tempo decks willing to pay twice and swing always, and worthless to anyone who wanted optionality.