Isildur cut the Ring from Sauron's hand with the hilt-shard of his father's sword, and Sauron himself was overthrown.

Isildur's Fateful Strike

(You may cast a legendary instant only if you control a legendary creature or planeswalker.)
Destroy target creature. If its controller has more than four cards in hand, they exile cards from their hand equal to the difference.

The Lord of the Rings: Tales of Middle-earth

Tales of Middle-earth Promos

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.

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.