You're right in noticing that!
The Fatal Decrees will be custom hunts published after GUT V3 (GUT's final version, which is coming somewhere in April) where you are instructed to hunt down that Pillar at the behest of CAIN's Authority. You'll then be able to execute them for good, earning the rank of Pillar yourself, or spare them, incurring a secret and very difficult boss fight.
Stay tuned!