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.
Sanctum of the Sun
Legendary Land
(Transforms from Azor's Gateway.)
: Add X mana of any one color, where X is your life total.
As soon as Jace laid eyes on the great sphinx's chamber, the shards of memory began sliding into place.
Yeong-Hao Han
The List (PLST)
• #RIX-176Rulings
If a card in exile has {X} in its mana cost, X is considered to be 0.
(wotc • 2018-01-19)
The mana value 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 mana values.
(wotc • 2018-01-19)
If an object has no mana cost, its mana value is 0. A mana value of 0 can still help unlock Azor's Gateway.
(wotc • 2018-01-19)
The ability of Sanctum of the Sun is a mana ability. It doesn't use the stack and can't be responded to.
(wotc • 2018-01-19)
The cards you exile from your hand are exiled face up.
(wotc • 2018-01-19)
Rulings
If a card in exile has {X} in its mana cost, X is considered to be 0.
(wotc • 2018-01-19)
The mana value 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 mana values.
(wotc • 2018-01-19)
If an object has no mana cost, its mana value is 0. A mana value of 0 can still help unlock Azor's Gateway.
(wotc • 2018-01-19)
The ability of Sanctum of the Sun is a mana ability. It doesn't use the stack and can't be responded to.
(wotc • 2018-01-19)
The cards you exile from your hand are exiled face up.
(wotc • 2018-01-19)