After some testing, it seems the dmg proc on feareater is ~112 on a vorpal if the target is over 5k hp or is otherwise immune to the instakill (possibly also on a save?). That means it adds an average of ~5.6 dmg per swing. That's less than disintegration (~9.15), a lvl 4 elemental dmg augment (7), meteoric star ruby on a lvl 34 weapon (~9.52), or even the dmg from regular nightmares (22.5)!
The only reason I could see to slot this is for the instakill vs trash on high skulls. If you have even somewhat decent dps and don't run above r4, I would avoid this purple raid augment and rather slot eye of dolurrh (10d6 evil dmg and ghost touch), deconstructor, disintegration, a regular elemental dmg augment (can trade for 8 or 9d6 from randall), or even a spellpower augment.
The only reason I could see to slot this is for the instakill vs trash on high skulls. If you have even somewhat decent dps and don't run above r4, I would avoid this purple raid augment and rather slot eye of dolurrh (10d6 evil dmg and ghost touch), deconstructor, disintegration, a regular elemental dmg augment (can trade for 8 or 9d6 from randall), or even a spellpower augment.