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.

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.

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).