Phyrexian Tower

The colorless mana on the first ability is almost an afterthought: what this land sells is the second line, where a creature you no longer need becomes two black mana at no cost beyond the tap. That is the trick. Most ritual effects pay you in mana you spend immediately; this one converts a board presence you were going to lose anyway (a token, a creature that already triggered on its way in, a body about to die in combat) into acceleration that stays on the battlefield. The land doesn't deplete, so the conversion is repeatable across turns, and the sacrifice is built into a mana ability, which means it resolves without the stack and can't be answered the way a sacrifice spell can. That timing is the whole reason it persists: it slots the sacrifice into windows where an opponent has no response, and it pairs the engine with whatever wants creatures dead for value on its own terms. The legendary supertype is what keeps the rate from running away; you get one, not four, and the deck has to earn the bodies it feeds the land. For decks built around expendable creatures, it is less a land than a permanent sacrifice outlet that happens to fix mana, and the design has held up precisely because the constraint (legendary, one creature at a time, two black only) keeps the ceiling honest while the floor stays useful.