Urza's Ruinous Blast
Legendary Sorcery
Oracle Text
(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.) Exile all nonland permanents that aren't legendary.Rulings (2)
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Wizards of the CoastYou can't cast a legendary sorcery unless you control a legendary creature or a legendary planeswalker. Once you begin to cast a legendary sorcery, losing control of your legendary creatures and planeswalkers won't affect that spell.
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Wizards of the CoastOther than the casting restriction, the legendary supertype on a sorcery carries no additional rules. You may cast any number of legendary sorceries in a turn, and your deck may contain any number of legendary cards (but no more than four of any with the same name).