Fist of Suns

If you pay white mana blue mana black mana red mana green mana rather than pay the mana cost of a spell with X generic mana in its mana cost, the only legal choice for X is 0.

If you cast a spell for which mana of any color can be spent to cast it, you may cast it for Fist of Suns's alternative cost and still spend any color of mana to cover that cost.

Fist of Suns's ability is an alternative cost to cast a spell. You can't combine this with other alternative costs, such as flashback. You can pay additional costs, such as kicker, in addition to this alternative cost.