Ashiok's Erasure

• WOTC Official Reminder Textofficial rules
  Flash — You may cast this spell any time you could cast an instant.

• WWizards of the Coast2020-01-24
  Official Ruling
  If there is no exiled card (perhaps because the exiled spell was a copy of a spell), or if the exiled card leaves exile somehow, Ashiok's Erasure won't stop players from casting spells.
• WWizards of the Coast2020-01-24
  Official Ruling
  If Ashiok's Erasure has exiled an adventurer card, your opponents can't cast spells with the same name as the creature, but may cast their Adventures.
• WWizards of the Coast2020-01-24
  Official Ruling
  The exiled spell isn't countered, but it won't resolve. This works against spells that can't be countered.
• WWizards of the Coast2020-01-24
  Official Ruling
  If Ashiok's Erasure has exiled a split card, your opponents can't cast a spell with the same name as either half of that card.
• WWizards of the Coast2020-01-24
  Official Ruling
  If Ashiok's Erasure leaves the battlefield before its enters-the-battlefield ability has resolved, its last ability triggers and resolves with no effect, then its enters-the-battlefield ability exiles the target spell for the rest of the game.

Rulings via Scryfall