Herald of Eternal Dawn

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

• WWizards of the Coast2024-11-08
  Official Ruling
  Other circumstances can still cause you to lose the game. For example, you will lose a game if you concede or if you're penalized with a Game Loss or a Match Loss during a sanctioned tournament due to a tournament rules infraction.  
• WWizards of the Coast2024-11-08
  Official Ruling
  You can't pay more life than you have, even if you won't lose the game.
• WWizards of the Coast2024-11-08
  Official Ruling
  No game effect can cause you to lose the game or cause any opponent to win the game while you control Herald of Eternal Dawn. It doesn't matter whether you have 0 or less life, you're forced to draw a card while your library is empty, you have ten or more poison counters, you have no other choice out of your Demonic Pact, an opponent reaches the Maze's End, or so on. You keep playing.  
• WWizards of the Coast2024-11-08
  Official Ruling
  If you control Herald of Eternal Dawn in a Two-Headed Giant game, your team can't lose the game and the opposing team can't win the game.

Rulings via Scryfall