Spawning Bed

The colorless mana it taps for is the appetizer; the sacrifice ability is the whole meal. Spend six mana, tap it, and crack the land, and you trade a mana source for three Eldrazi Scions, each of which sacrifices for one colorless mana of its own. So the land does not directly refund mana on the activation: it hands you three bodies, and any mana comes only if you then start sacrificing those tokens as a separate action. That two-step is the design. The land defers value rather than generating it, sitting in play producing colorless until you decide the late game has arrived, at which point it converts itself into a wad of Scions you can spend piecemeal. The tokens matter beyond their stats: three chump blockers, three pieces of sacrifice fuel for a token-fueled engine, three triggers for anything that cares about creatures entering or dying, or up to three colorless mana banked back if you crack them all at once. Producing only colorless is the cost of admission, threading no color into a deck that may need it; in exchange the land asks nothing of its host except a long enough game and a willingness to commit six mana to the cash-out. It is slow by design, a sink for the mana your other lands are wasting once the board has stabilized, built for Eldrazi decks that already speak colorless and devotion to nothing in particular.