Cactarantula

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.

Cactarantula’s last ability resolves before the spell or ability that caused it to trigger. It resolves even if that spell is countered.

Cactarantula still costs only 1 generic mana less to cast if you control multiple Deserts.