Prophecy Inescapable

(You may cast a legendary instant only if you control
a legendary creature or planeswalker.)
This spell costs {4} less to cast if an opponent has
their commander on the battlefield.
Prophecy Inescapable deals 4 damage to each
creature and planeswalker you don't control. If
at least 10 or more permanents are destroyed this
way, exile them.