Isildur's Fateful Strike

• WWizards of the Coast2023-06-16
  Official Ruling
  Isildur's Fateful Strike is the first legendary instant. It is subject to the same casting restrictions as existing legendary sorceries, as listed in the following rulings.
• WWizards of the Coast2023-06-16
  Official Ruling
  You can't cast a legendary instant unless you control a legendary creature or a legendary planeswalker. Once you begin to cast a legendary instant, losing control of your legendary creatures and planeswalkers won't affect that spell.
• WWizards of the Coast2023-06-16
  Official Ruling
  Other than the casting restriction, the legendary supertype on an instant carries no additional rules. You may cast any number of legendary instants in a turn, and your deck may contain any number of legendary cards (but no more than four of any with the same name).

Rulings via Scryfall