MTG Treachery

The Seer’s unveiling triggered ability goes on the stack without a target. When you reveal a nonland card during its resolution, its reflexive triggered ability triggers and you pick a target to be dealt damage. You’ll know the mana value of the revealed nonland card and how much damage will be dealt as you choose your target.
The X in the unveiling triggered ability has the same value as the X paid in the Unveil ability. This is pretty easy to derive since there is no other source of X.
If X is 0, you won’t scry at all. Any abilities that trigger whenever you scry won’t trigger.
Once The Seer’s unveiling triggered ability begins to resolve, no player may take other actions until it’s done. Notably, opponents can’t try to change your library after you scry but before you reveal the top card of your library.

If the revealed card in your library has {X} in its mana cost, X is considered to be 0.
The converted mana cost of a split card is based on the combined mana cost of its two halves.
An identity card has no color.