Azor's Gateway
Legendary Artifact
, 
: Draw a card, then exile a card from your hand. If cards with five or more different mana values are exiled with Azor's Gateway, you gain 5 life, untap Azor's Gateway, and transform it.
Illustrated by
The List#RIX-176MythicEnglishLegality
Rulings
The cards you exile from your hand are exiled face up.
The converted mana cost of a split card, such as a card with aftermath from the Amonkhet block, is based on the combined mana cost of its two halves. A split card doesn’t have two converted mana costs.
If a card in exile has 
in its mana cost, X is considered to be 0.
If an object has no mana cost, its converted mana cost is 0. A converted mana cost of 0 can still help unlock Azor’s Gateway.
For more information on double-faced cards, see the Ixalan mechanics article (http://magic.wizards.com/en/articles/archive/feature/ixalan-mechanics).
The ability of Sanctum of the Sun is a mana ability. It doesn’t use the stack and can’t be responded to.