A 6-mana Temur legend from Avatar: The Last Airbender that gives every instant and sorcery in your graveyard flashback during your turn — turning a spent hand into a second one.
Spellslinger decks. A graveyard full of cheap cantrips and removal spells becomes a second hand the moment Iroh resolves, rewarding a deck built around casting a lot of spells.
Ramp-and-value Temur shells. Recasting ramp spells for free value on top of their first cast is a strong mana-efficiency boost for a midrange deck.
Prowess and instant/sorcery matters. Every recast off the graveyard also triggers prowess and similar payoffs a second (or third, or fourth) time.
How it does in Commander
As the commander
As the commander it anchors a spellslinger deck directly, since guaranteed access to the graveyard-wide flashback from the command zone rewards building the whole 99 around a low curve of efficient, recastable spells.
In the 99
A strong late-game value engine even outside a full spellslinger build, turning whatever instants and sorceries a Temur deck has already cast into a genuine second wind.
Formats it's strong in
Spellslinger and prowess decks. The effect is a direct payoff for casting a lot of cheap instants and sorceries, which those decks are already doing.
Long, grindy games. The longer a game runs, the fuller the graveyard gets, and the more value Iroh's ability produces.
Formats it's weak in
Low spell-count decks. With few instants and sorceries in the graveyard, the ability has little to work with.
Fast, aggressive decks. Six mana for an ability that pays off over a long game is a poor fit for a deck trying to end things quickly.
Synergistic cards
Brainstorm — A cheap cantrip that becomes repeatable card selection for as long as Iroh is on the battlefield.
Lightning Bolt — Efficient removal that becomes a recurring answer once it's sitting in the graveyard under Iroh.
Cultivate — Ramp that can be recast for its full mana cost later in the game for extra land drops on top of its first cast.