Invoke Calamity

If you cast a spell without paying its mana cost, you can't choose to cast it for any alternative costs. You can, however, pay any additional costs. If the spell has any mandatory additional costs, you must pay those.

The spells are cast one after the other during the resolution of Invoke Calamity. The one you cast second will be the first one to resolve.

If the spell has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.