Fateful Hour is a mechanic introduced in Dark Ascension that exists only on 7 cards, 5 white and 2 green.
Fateful Hour triggers when you have 5 or less life. The list of cards with Fateful Hour is here. Personally, I find it to be an interesting mechanic, but it’s not very consistent, and most of the cards with Fateful Hour aren’t very impressive. The only ones that see play are Thraben Doomsayer and Gather the Townsfolk.