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Flashback will probably still be the most dominant way to take advantage of Sevinne, with a much greater saturation of cards with the mechanic (plus cards that favorably interact with flashback, like Burning Vengeance or Secrets of the Dead)

Behemoth Sledge Brawler's Plate Bronzeplate Boar Ceremonial Groundbreaker Chariot of Victory Colossal Dreadmask Cori-Steel Cutter Dragon Throne of Tarkir Dwarven Hammer Eater of Virtue Embercleave Forebear's Blade Gate Smasher Goblin Morningstar Haunted Cloak Helm of Kaldra Hero's Heirloom Hexgold Halberd Horned Helm Kaldra Compleat Leyline Axe Lightning Spear Loxodon Warhammer Meteoric Mace Novel Nunchaku O-Naginata Power Fist Ramosian Greatsword Raven Clan War-Axe Ring of Kalonia Samurai's Katana Scepter of Celebration Shadowspear Spiked Ripsaw Sword of Vengeance Sylvok Battle-Chair T-45 Power Armor Team Pennant Tenza, Godo's Maul The Reaver Cleaver Vibranium Strike Gauntlets Vorrac Battlehorns

Is it one of the key cards in a format, and, if so, is it a four-of in most decks

The most common line of play will probably be to play her and draw a card for her first trigger, then transform her the following turn before she attacks