Sharuum the Hegemon is just outdated unless you either use Sculpting Steel combos or go deep with flicker shenanigans, neither of which this deck does
Island (268) (Phyrexian) [Phyrexia: All Will Be One] Set: Phyrexia: All Will Be One Type: Basic Land Island Rarity: Common

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That might be a combo piece, a mana engine, a lock piece, or the exact answer you need next turn
Until end of combat, you don't lose this mana as steps end." (10/2/2025) Mana from firebending abilities isn't lost until you leave combat and go to your second main phase
Then he can -7 as an ultimate and then your Mountains will Lightning Blast on arrival to the battlefield