Urza, Powerstone Prodigy

Looting is one of blue's oldest engines, but the design here inverts which half of the loot you care about. Normally "draw, then discard" asks you to bleed off a card you did not want; the discard is the tax you pay for the dig. This reframes the discard as the payoff: pitch an artifact and you bank a tapped Powerstone, colorless ramp that can't cast a nonartifact spell but still pays for artifacts and activated abilities like Urza's own loot. A hand clotted with dead metal stops being clutter and becomes fuel. The once-per-turn clamp on the trigger is the load-bearing restriction. Without it, a hand full of artifacts and any second discard outlet would spiral into extra tokens; with it, the engine grinds out exactly one token a turn, always tapped, always earmarked, so the card is steady rather than explosive. The on the activation matters too: unlike a free looter, each loop costs a mana, which is precisely what the Powerstones are there to provide, closing the loop into something self-funding rather than free. The 1/3 with vigilance is not a clock; it is a body designed to keep attacking without ever taking the engine offline. Everything points the same direction: a self-sustaining source of colorless mana that an artifact deck can keep feeding, where the loot smooths the draw, the discard converts excess into ramp, and the ramp bankrolls the next artifact you loot away.