Urza's Ruinous Blast
Legendary Sorcery
(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.)
Exile all nonland permanents that aren't legendary.
Centuries ago, one man's vengeance plunged the world into ice and darkness.
Noah Bradley
Dominaria (DOM)
• #39Rulings
You 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.
(wotc • 2018-04-27)
Other 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).
(wotc • 2018-04-27)
Rulings
You 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.
(wotc • 2018-04-27)
Other 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).
(wotc • 2018-04-27)