Skoa, Embermage

In Commander, Skoa, Embermage is a passive damage engine that scales cleanly with spell-heavy decks — the more instants and sorceries you were already planning to cast, the more free damage it staples onto every turn. Pauper is where it arguably shines hardest: the format is full of cheap cantrips and removal, and a three-mana 2/3 that taxes the opponent's life total every time you cycle a Preordain is a legitimate threat. Legacy and Vintage have access to the density of cheap spells to make Skoa, Embermage dangerous, but both formats also have the removal to answer a 2/3 before it generates meaningful value, which caps its ceiling in competitive play. Modern is the tightest fit — the bar for three-drop creatures is high, and decks that want this effect usually have more efficient options available.