Seal of the Guildpact

• WWizards of the Coast2015-11-04
  Official Ruling
  If a spell you cast has 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 and you've chosen red and green, you could choose 5 as the value of X and pay to cast the spell.
• WWizards of the Coast2015-11-04
  Official Ruling
  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.
• WWizards of the Coast2015-11-04
  Official Ruling
  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.
• WWizards of the Coast2015-11-04
  Official Ruling
  You must choose two different colors.
• WWizards of the Coast2015-11-04
  Official Ruling
  The cost reduction can apply to alternative costs such as evoke costs.
• WWizards of the Coast2015-11-04
  Official Ruling
  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.

Rulings via Scryfall