If no player has an opened breach I or II, Tempest Sigil may be used to allow any player to prep a spell out of turn (Any player destroys an opened I or II breach and returns any spells prepped to that breach to their hand. That player gains a Sigil breach and places it where the destroyed breach was. Then, that player may prep a spell from their hand to a breach.).
Tempest Sigil cannot be used to place a Sigil Breach without also destroying a I or II breach. It cannot replace a breach that was previously destroyed by a different effect, nor can it replace a breach that never existed.
