Fateful Hour is an ability word. While the comprehensive rules do govern how these abilities work, there are not dedicated rules in the comprehensive rules about Fateful Hour. The specific workings of this mechanic are printed on the cards themselves. However, some additional rules knowledge about the mechanic can be useful.

Fateful Hour ‘works’ when you have a life total of five or less. If your life total is six or more, then Fateful Hour abilities will not do anything.

How Fateful Hour affects individual spells and abilities depends on the exact ability that this ability word appears in. On some instant and sorcery spells, Fateful Hour will change what happens when the spell resolves. These Fateful Hour abilities use the word ‘instead,’ and in these instances are replacement effects. For example, Gather the Townsfolk‘s fateful hour ability will change what happens when the spell resolves. Instead of getting two token creatures, you get five.

On other spells, Fateful Hour will give you a bonus effect if your life total is five or below. Clinging Mists prevents all combat damage in the current turn, regardless of how much life you have. However, if you have five or less life, the Fateful Hour effect will also tap all attacking creatures and keep these creatures from untapping during their controller’s next untap step.

There are also a few Fateful Hour static abilities. These abilities are simply on or off depending on your life total. With Thraben Doomsayer, if your life total is five or less, other creatures you control will get +2/+2. If your life total is six or more, this ability does nothing. Keep in mind that these static abilities will continually check your life total to determine if they are ‘on’ or ‘off.’