Seal of the Guildpact

2015-11-04 If a spell you cast has {X} in its mana cost, you choose the value of X before calculating the spell's total cost. For example, if that spell's mana cost is {X}{R}{G} and you've chosen red and green, you could choose 5 as the value of X and pay {3}{R}{G} to cast the spell.
2015-11-04 If there are additional costs to cast a spell, or if the cost to cast a spell is increased by an effect (such as the one created by Thalia, Guardian of Thraben's ability), apply those increases before applying cost reductions.
2015-11-04 Seal of the Guildpact's last ability can't reduce the amount of colored mana you pay for a spell. It reduces only the generic component of that cost.
2015-11-04 Seal of the Guildpact's last ability doesn't change the mana cost or mana value of any spell. It changes only the total cost you pay.
2015-11-04 The cost reduction can apply to alternative costs such as evoke costs.
2015-11-04 You must choose two different colors.

mtg.wtf