What it does
Fateseal lets you look at the top card (or cards) of an opponent's library and choose to either leave it there or put it on the bottom. It's essentially Scry, but aimed at an enemy's deck instead of your own—you're denying them good draws rather than fixing your own.
The key distinction people miss: Fateseal targets opponents, Scry targets yourself. They function identically otherwise, but they're separate keywords and effects, so cards referencing one don't trigger off the other. Fateseal N means you affect the top N cards.
In Commander, Fateseal is weaker than in 1v1 because you can only mess with one opponent at a time while three players keep drawing freely. It's most valuable when locking down a single threatening opponent, or with repeatable engines like Jace, the Mind Sculptor that grind their draws every turn.

