Centuries ago, a quest to harness time became a spiral into chaos.

Karn's Temporal Sundering

(You may cast a legendary sorcery only if you control a legendary creature or planeswalker.)
Target player takes an extra turn after this one. Return up to one target nonland permanent to its owner's hand. Exile Karn's Temporal Sundering.

Dominaria

Dominaria Promos

If the target player or target nonland permanent is an illegal target as Karn’s Temporal Sundering resolves, the other target is affected as normal and Karn’s Temporal Sundering is exiled. If both targets are illegal, Karn’s Temporal Sundering doesn’t resolve and isn’t exiled.

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).

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.