As Gideon kept the brood lineages at bay, Chandra incinerated the bound Eldrazi titans.

Fall of the Titans

Surge {X}{R} (You may cast this spell for its surge cost if you or a teammate has cast another spell this turn.)
Fall of the Titans deals X damage to each of up to two targets.

The List

Oath of the Gatewatch

Surge {X}{R} (You may cast this spell for its surge cost if you or a teammate has cast another spell this turn.)
Fall of the Titans deals X damage to each of up to two target creatures and/or players.

Oath of the Gatewatch Promos

Casting a spell for its surge cost doesn't change its mana cost or its mana value.

Fall of the Titan's mana value is based on its mana cost of {X}{X}{R}, even if you're casting it for its surge cost. For example, if you cast Fall of the Titans for its surge cost and choose 4 for X, its mana value will be 9.

For some cards, surge represents only an alternative cost, a discount that applies if you or a teammate has cast another spell this turn. Other cards, like Reckless Bushwhacker, have additional abilities or effects if you paid the surge cost to cast the spell.

If an instant or sorcery spell cast for its surge cost is copied, the copy is also considered to have had its surge cost paid.

The other spell that you or a teammate cast can be one that's resolved, one that was countered, or (for instants with surge) one that's still on the stack.