Fateful hour

Fateful hour is an ability word introduced in Dark Ascension.

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. So 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—so gaining life with Thraben Doomsayer on the table might cause your creatures to get smaller in the middle of a turn.

Other fateful hour abilities may be triggered abilities, static abilities, or abilities that change what an instant or sorcery spell does.