Cactarantula

• WWizards of the Coast2024-04-12
  Official Ruling
  If a spell or ability targets Cactarantula more than once, its last ability still triggers only once.
• WWizards of the Coast2024-04-12
  Official Ruling
  Cactarantula still costs only less to cast if you control multiple Deserts.
• WWizards of the Coast2024-04-12
  Official Ruling
  Cactarantula’s last ability resolves before the spell or ability that caused it to trigger. It resolves even if that spell is countered.
• WWizards of the Coast2024-04-12
  Official Ruling
  Once you announce that you’re casting Cactarantula, no player may take actions until the spell has been paid for. Notably, opponents can’t try to change whether you control a Desert.

Rulings via Scryfall