Slayers' Stronghold

The colorless tap is the polite half; the real text is a land that pumps and grants haste, an effect that ordinarily costs you a card in hand. What makes this dangerous is that it survives sweepers. Most attack-step enablers, the Fervor variants and the various anthems, sit in the same blast radius as the creatures they buff, so an opponent answering your team also answers your reach. A land does not die to Wrath of God, so the activation comes back the turn after a reset, ready to give a freshly cast threat haste and vigilance and a swing it should not have had yet. The vigilance clause is the quiet part: it turns the buff into a tempo swing that does not lower your guard, letting an attacker also stay home for the crackback. The cost is the obvious tax for tucking a combat trick into your manabase: it taps for colorless only, so leaning on it for fixing punishes a greedy draw, and the activation wants two specific colors and the land's own tap, which means the turn you fire it you are not also ramping with it. That is the trade across the small cycle of utility lands that buff a creature: you accept a slightly worse mana source to fold a recurring, sweeper-proof effect into a slot that costs you no cards.