Infernal Phantom

• WOTC Official Reminder Textofficial rules
  Eerie — Triggered ability triggers whenever an enchantment enters the battlefield under your control and whenever you fully unlock a Room.

• WWizards of the Coast2024-09-20
  Official Ruling
  If a permanent with an eerie ability enters at the same time as one or more enchantments, its ability will trigger for each of those enchantments.
• WWizards of the Coast2024-09-20
  Official Ruling
  An ability that triggers "whenever you fully unlock a Room" triggers when a door becomes unlocked and the other door of that Room is already unlocked, or when both doors of that Room become unlocked simultaneously.
• WWizards of the Coast2024-09-20
  Official Ruling
  Use Infernal Phantom's power as it last existed on the battlefield to determine how much damage it deals due to its last ability.

Rulings via Scryfall