Altar Golem

The cost of attacking is paid in the same currency it counts. Its power and toughness are a live tally of every creature on the battlefield, so the bigger it grows, the wider the board everyone has committed: and that committed board is exactly what the untap clause taxes. It never untaps on its own, and freeing it to swing again demands tapping five untapped creatures you control. That circular accounting is the design problem the card poses. The five bodies you tap do not leave the battlefield, so the Golem stays the same size; what you spend is their utility for the turn. A board wide enough to make this thing enormous is also a board you would rather be attacking with than tapping down to wheel a single trampler back into action. Run it into a stalled, creature-clogged stand-off and it arrives as a real threat with trample to push damage through; reset it for a second swing and you have frozen five of your own attackers to do it. The cost is steep enough that the ability reads as a ceiling rather than a recurring plan, a once-per-turn emergency lever rather than something you lean on. It belongs to the family of artifact bodies whose stats are a readout of the board state rather than fixed numbers, where the answer to "how big is it" is always "it depends, and on whose turn are you asking."