Colossal Rattlewurm

Colossal Rattlewurm has flash as long as you control a Desert.
Trample
{1}{G}, Exile this card from your graveyard: Search your library for a Desert card, put it onto the battlefield tapped, then shuffle.

Outlaws of Thunder Junction Promos

Outlaws of Thunder Junction

Colossal Rattlewurm has flash as long as you control a Desert.
Trample
{1}{G}, Exile Colossal Rattlewurm from your graveyard: Search your library for a Desert card, put it onto the battlefield tapped, then shuffle.

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.