Ozai, the Phoenix King

The mana-retention clause is where this stops being a curve-topper and becomes a mana bank. Kruphix, God of Horizons taught the trick years ago: any mana you would otherwise lose becomes red and stays in your pool, so a full six can carry into your opponent's turn without asking for extra infrastructure to hold it. That same clause arms the payoff. Keep six or more floating and the 7/7 gains flying and indestructible, which inverts the usual pressure to spend down: leftover mana is normally dead value, but here it is protection you re-buy every phase you decline to empty out. Firebending 4 is the fuel line, generating the exact red mana the retention clause preserves, so the deck it wants is one built to make and hoard large pools rather than dump them into the board. The tension is that the threshold costs tempo: chase it and you develop nothing while your mana sits idle, and any removal that resolves while your pool dips below six meets an ordinary, killable body with no ceiling protecting it. Haste and trample mean the raw stats already threaten to close a game the turn it lands; the conditional flying and indestructible are what make it hard to answer once the pool is stocked. It reframes mana as a resource to hold rather than one to spend, and punishes you the moment you forget it.