Magvara, the Frozen Caldera

Magvara, the Frozen Caldera enters the battlefield tapped.
{t}: Add {u}.
Whenever you cast a red spell, you may pay {1}{u}. If you do, transform Magvara the Frozen Caldera.
In the heart of the Frozen God's domain, Xarion sparked Kaldheim's red mana. He did not know that...