Grim Monolith

Three colorless from a two-mana rock is only one mana ahead of where you started, and the non-untap clause is the whole point: this is a battery, not an engine. Play it, tap it once, and you have spent your acceleration in a single burst rather than dripping value across a game. The four-mana untap cost means a second tap inside the same turn is a real investment, so the card asks you to convert the tempo into something explosive the moment you have it: a bomb several turns ahead of curve, or a combo piece that wants exactly one big push. That single-burst profile found its definitive partner in Power Artifact, which lowers the untap cost enough to make the Monolith generate infinite colorless mana, the interaction that has kept it relevant in eternal combo long after its days in rotation closed. The non-untap restriction is the friction that earns the rate: it forces you to commit to the speed it grants rather than hoarding it, and it forecloses the kind of repeatable, every-turn ramp that a Sol Ring offers. Reserved List status froze its supply where it sits, but the design logic was timeless from the start. A fast rock that demands you spend the tempo it offers right away is a cleaner, more honest piece of acceleration than its modest output line suggests.