The bogeyman's Terrible Rejuvenation ability grants fast healing 5 while someone is affected by a fear effect while within his Deepest Fear aura.
The implacable stalker template's Terrifying Inevitability grants fast healing equal to its Hit Dice while the implacable stalker can see or hear someone suffering from a fear effect. (That among many other effects of Terrifying Inevitability.)
So my question is do the two sources of conditional fast healing stack, or would the Terrifying Inevitability (17 HD) just override the Terrible Rejuvenation 5?
No, fast healing from different sources doesn't stack, you just get the best of.
Alright. So if they don't stack, the Terrifying Inevitability just replaces the Terrible Rejuvenation. They both trigger by fear effects, so there's never a case of terrible rejuvenation triggering without terrifying inevitability triggering. In that case, I may leave Terrible Rejuvenation off the stat block entirely.
I'll probably give the monster 1 or 3 more Hit Dice (for 18 or 20), because I like round numbers and Fast Healing 17 seems weird to me.
Hmm. Actually, Terrible Rejuvenation works while a creature with fear is in the deepest fear aura, while Terrifying Inevitability requires the implacable stalker to see or hear the creature. Someone could be invisible in the aura, I suppose.