Titans' Nest
Enchantment
At the beginning of your upkeep, surveil 1. (Look at the top card of your library. You may put that card into your graveyard.)
Exile a card from your graveyard: Add 
. Spend this mana only to cast a spell that's one or more colors without 
in its mana cost.
Cliff Childs
Rulings
If you don’t put the card you look at into your graveyard, it remains on top of your library. You’ll draw it during your draw step unless something else moves it.
(wotc • 2020-04-17)
Mana produced by Titans’ Nest can be used to cast a spell that has {X} in an additional or alternative cost.
(wotc • 2020-04-17)
Rulings
If you don’t put the card you look at into your graveyard, it remains on top of your library. You’ll draw it during your draw step unless something else moves it.
(wotc • 2020-04-17)
Mana produced by Titans’ Nest can be used to cast a spell that has {X} in an additional or alternative cost.
(wotc • 2020-04-17)