Reset

Designed as a pure mana instrument, with a timing restriction that defines the entire card. The opponent's-turn-after-upkeep clause is the structural pressure valve: you cannot untap on your own turn for a Time Walk effect, and you cannot fire it during your own upkeep to dodge a cost. What you can do is wait until the opponent passes priority on their turn, then untap your lands and pour a second turn's worth of mana into something enormous at instant speed. That function is exactly the engine the classic Solidarity decks run on: chain Reset alongside High Tide and the format's blue mana doublers to manufacture a mana pool large enough to draw the deck and kill on the opponent's end step. The double-blue cost and the surgical timing kept it niche, but the function is anything but vestigial. Where Mana Vortex and the Power Surge family punish players for the lands they leave untapped, Reset rewards: it converts the opponent's turn into a windfall, provided you built a deck that wanted a second mana pool at exactly that window. It remains one of the cleanest illustrations of how Legends treated phase structure as a design lever, and unlike most cards built on that kind of timing trick, the interaction it depends on still works precisely as printed.