Mystic Forge

Casting spells straight off your library is one of the oldest tempting knobs in the game, and the reason it stays rare is obvious once you build around it: it turns your deck into a second hand that never runs out. What keeps this one honest is the read on its permission slip. It does not let you cast everything you see, only artifacts and colorless spells, which quietly restricts the enabler to artifact-dense shells where nearly every card qualifies. The tap ability closes the loop: a colored card stuck on top blocks the whole engine, so exiling it for a life is the release valve that turns dead draws back into live ones. That combination (see what's coming, cast the artifacts, burn away the rest) makes it a genuine engine rather than a card-advantage rounding error. It also makes it dangerous in a way that colored-cost velocity is not, because a mana rock that produces more than it costs feeds directly back into more castable spells, and any repeatable "reduce artifact costs to nothing" effect turns the whole library into a chain you cast for free. The design is a bet that the artifact-only clause is a strong enough leash: fine in a fair deck grinding value, catastrophic the moment the count of zero-cost artifacts and net-positive rocks tips past a threshold.