Mana Vault

The original Power Nine-adjacent fast mana, and the cleanest statement of what "fast mana" actually costs. Three colorless for one was a rate the game has spent decades apologizing for, so the design built the apology into the card itself: the artifact doesn't untap on its own, the buyback is a punishing four mana, and if you forget about it you take a point in your draw step. The whole card is a structured loan. You get a turn ahead on mana now, you pay for it in tempo and life later, and the upkeep clause exists so the loan has a redemption window rather than rotting on the battlefield forever. What makes the design hold up is that every clause does a specific piece of balancing work: the no-untap clause prevents it from being a Sol Ring, the optional four-mana untap prevents it from being a dead draw once you have spent it, and the one-damage trigger prevents you from parking it tapped as a free permanent for graveyard or artifact-count purposes. Grim Monolith is the direct descendant, keeping the same four-mana untap but raising the casting cost so the rate sits a little further from free. This is the template, and the template is why colorless ritual-rate mana rocks have carried built-in drawbacks ever since.