It seems pretty well done once you fill in description, flavor, and tactics.
Mayfair said:Foregathering Spirits are a form of undead which appear as a swirling yellow mist about the size of a man. When looking at the mist, each individual sees a vision that is so horrible, it cannot be described. The creature actually takes on the appearance of the character’s greatest fears.
If any characters encounter a Foregathering Spirit, they must immediately save vs. spells or be affected by the equivalent of a Fear spell. The affected character will flee as far and fast as possible for 2-5 rounds.
Shouldn't it be a fear aura instead? Frightful presence usually requires some action to trigger it (like attacking; see dragons).
Fear Aura (Su): Foregathering spirits are surrounded by a X-foot-radius fear aura. Affected creatures must succeed on a DC X Will save or be affected as though by a fear spell (caster levelX). A creature that successfully saves cannot be affected again by the same foregathering spirit’s aura for 24 hours. Other foregathering spirits are immune to the aura. The save DC is Charisma-based.
Either way, doesn't really matter.