Does Sigil of the Storm target? If it does, can I react to the trigger by recycling the chosen rune for Power? That seems weird.

Sigil of the Storm is receiving errata using a new wording to make sure that it doesnʼt target. This uses a new wording, “must,ˮ that indicates an action that must be taken. None of the choices made for such an ability are targets, and they are chosen as the ability resolves. When you conquer here, you must recycle one of your runes.