A creature whose toughness is being increased by Thraben Doomsayer's fateful hour ability loses that bonus as soon as its controller has 6 or more life (or as soon as Thraben Doomsayer leaves the battlefield). If damage marked on that 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.

If Thraben Doomsayer's activated ability resolves while you have 5 or less life, the creature token will be 3/3 as it enters.