Solfatara

Solfatara is legal in Commander, Legacy, Vintage, Pauper, and Oathbreaker — but the formats where it actually sees play are niche. In Commander, one untap denial hits a single opponent at a table of four, which makes Solfatara most effective when targeting the player with the best board state or when looped repeatedly with recursion engines. In Pauper it's a common, so the ceiling for abuse is real if you can recur it, though dedicated stax lists usually prefer permanents that generate repeated lock pieces. Legacy and Vintage have faster clocks and tighter constraints on three-mana do-nothings, so Solfatara rarely competes there.