Fateful hour

Fateful hour is an ability word introduced in Dark Ascension.

Description
Fateful hour abilities work as long as you have 5 life or less. As soon as your life total drops to 5 or less, they immediately kick in. Thus, you could pay some life to an Immolating Souleater in the middle of combat to suddenly give your team a boost with Thraben Doomsayer. As soon as your life total becomes 6 or higher, fateful hour abilities stop working—thus, gaining life with a Thraben Doomsayer in play might cause your creatures to get smaller in the middle of a turn.

Of the eight cards with Fateful Hour, five of them are instants or sorceries that modify their resolution and three are creatures that have static abilities.

Fateful hour was probably the most unpopular named keyword of the original Innistrad block, so it didn't return in Shadows over Innistrad. It had a brief appearance in Modern Horizons, though.

Arguel's Blood Fast, Convalescent Care, and Second Chance functionally have Fateful Hour, two printed long before Innistrad and one printed some time after. Unlike the printed uses of Fateful Hour, all three of these are triggered abilities.

Rulings

 * Some fateful hour abilities that appear on instants and sorceries use the word "instead". The spells have an upgraded effect if you have 5 or less life at the time they resolve. You receive only the upgraded effect, not both effects.
 * Other instants and sorceries have fateful hour abilities that don't use the word "instead". They provide an additional effect if you have 5 or less life at the time they resolve.
 * Some fateful hour abilities (like Gavony Ironwright's) are static abilities. These abilities constantly check whether you have 5 or less life to see if the bonus applies. These abilities function only on the battlefield.
 * A creature whose toughness is being increased by a fateful hour ability loses that bonus as soon as its controller has 6 or more life. If damage marked on the creature is greater than or equal to its toughness after the fateful hour ability stops applying, the creature is destroyed and put into its owner's graveyard.