Sigil of the Empty Throne

• WWizards of the Coast2021-03-19
  Official Ruling
  An ability that triggers when a player casts a spell resolves before the spell that caused it to trigger, but after targets have been chosen for that spell. It resolves even if that spell is countered.
• WWizards of the Coast2021-03-19
  Official Ruling
  Because it's not on the battlefield yet, casting Sigil of the Empty Throne won't cause its own ability to trigger.

Rulings via Scryfall