Warden of the Wall

A mana rock that moonlights, and the whole bargain turns on which player's turn it is. On your own turns it sits inert: a colorless source nobody bothers to attack because there is nothing to attack. The moment the turn passes, it wakes into a 2/3 flier ready to block exactly when an attacker has committed, then folds back into a plain artifact when your turn comes around. The crucial detail is that even as a Gargoyle it keeps its tap ability, so on the opponent's turn it is genuinely doing both jobs at once: a creature in the air and a mana source you can still wring colorless out of. That dual identity reshapes its removal math in both directions. On your own turn it is not a creature, so it slips under your own sweepers and shrugs off any kill spell pointed at it during your main phase. The cost of that protection is that it spends the entire opposing turn as a creature, fully exposed to instant-speed removal or a board wipe resolved during the attack step, with no main-phase window where it shrinks back to safety on their side. The other cost is paid before any of this happens: it enters tapped, so the promised mana arrives a turn late and the ramp curve stalls. The trade is patience for value: a delayed colorless source that doubles as air defense an aggressor has to plan around.