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.

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).