Petrahydrox

• WWizards of the Coast2024-01-12
  Official Ruling
  Petrahydrox's ability triggers only if it's on the battlefield when it becomes the target of a spell or ability.
• WWizards of the Coast2024-01-12
  Official Ruling
  Petrahydrox's ability resolves before the spell or ability that caused it to trigger. It resolves even if that spell or ability is countered.
• WWizards of the Coast2024-01-12
  Official Ruling
  If the spell or ability that targets Petrahydrox has no other targets, it won't resolve (because it no longer has a legal target after Petrahydrox has returned to its owner's hand).

Rulings via Scryfall