Main Event Horizon

• WWizards of the Coast2022-10-07
  Official Ruling
  Because choosing a half of the alphabet is an additional cost, players will know which creatures are fated to be destroyed while pondering responses.
• WWizards of the Coast2022-10-07
  Official Ruling
  When determining what a name begins with, ignore all spaces and punctuation, including blank lines.
• WWizards of the Coast2022-10-07
  Official Ruling
  Creatures in languages that don’t use an alphabet that includes the letters A through M or N through Z are safe from Main Event Horizon. They won’t be destroyed no matter which of the alphabet was chosen.

Rulings via Scryfall