Darksteel Juggernaut

A self-scaling threat that turns your artifact count into both halves of its stat line, which means the card grows alongside the deck it wants to live in: the more rocks, equipment, and tokens on your side, the larger it swings. Because it is itself an artifact, it always counts itself, so the floor is a 1/1 even on an otherwise empty board; the body never collapses to nothing, it just stays small until the engine fills out. The indestructibility ties the package together: most board wipes destroy outright or scale damage by toughness, and this body sidesteps both, so the variable P/T never becomes a liability against removal that would punish a fragile X/X. The compulsory attack clause is the price for the rest. A creature that grows without bound and cannot be killed by conventional means would be oppressive if it could also block at will and pick its windows; forcing it into the red zone every combat hands the opponent a fixed plan, a predictable swing they can chump, race, or bounce even when they cannot destroy it. The trade is power for control: you get an indestructible threat whose ceiling rises with your build, and you give up the ability to hold it back. That tension keeps the rate honest. The card rewards stacking artifacts not as a synergy bonus but as the entire engine, since the threat is only as large as the board you have assembled to feed it.