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.

All of the permanents put onto the battlefield this way enter at the same time. If any have triggered abilities that trigger on something else entering the battlefield, they’ll see each other.

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).

For cards in your library with in their mana costs, X is considered to be 0.