Colossal Rattlewurm
Creature — Wurm
Colossal Rattlewurm has flash as long as you control a Desert.
Trample
, Exile this card from your graveyard: Search your library for a Desert card, put it onto the battlefield tapped, then shuffle.
Filip Burburan
Rulings
The game checks whether it is legal to cast a spell before costs are determined or mana abilities are activated while casting it. If you no longer control a Desert after that point, perhaps because you sacrificed all of your Deserts while activating mana abilities, this won't affect whether you can legally cast Colossal Rattlewurm.
(wotc • 2024-04-12)
Rulings
The game checks whether it is legal to cast a spell before costs are determined or mana abilities are activated while casting it. If you no longer control a Desert after that point, perhaps because you sacrificed all of your Deserts while activating mana abilities, this won't affect whether you can legally cast Colossal Rattlewurm.
(wotc • 2024-04-12)