Koilos Roc

Flash on a ramp creature is a rare pairing, and the specificity of what it makes is why the combination is allowed to exist. The Powerstone this leaves behind is not a Signet or a Mind Stone: its colorless mana can pay for activated abilities, triggers, and tax effects, but it can never be spent to cast a nonartifact spell. That walls the acceleration off from your blue spells and points it squarely at artifacts and the effects that ask for generic mana, which is what lets a Powerstone-maker come attached to a body with genuine defensive utility. Because the trigger fires on entry and the card can be held to end of turn, the play pattern inverts the usual ramp tension: instead of tapping out on your turn and passing with an empty board, you hold up five mana, ambush an attacker with a 3/3 flying blocker at instant speed, and bank a tapped Powerstone for next turn no matter how the combat resolves. The stone enters tapped, so it never accelerates the same turn the bird lands, keeping the flash-blocker line honest against anyone hoping to chain into an immediate artifact play. It sits in blue's tradition of flash fliers that manufacture an artifact on arrival, but the Powerstone's spending restriction reorients the whole card toward artifact-centric shells: the mana is only worth having if you have artifact spells and activated costs waiting to receive it.