Isildur's Fateful Strike

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).

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.