As an FD build, a backlash guest automatically becomes only 80% as efficient (because you take *0.8 damage). Add in the fact that Dread Fiend requires a save, and I'd say it's slightly inferior to the other 2 options.
Carnage backlash gains the same *0.8 FD penalty, it gives only 27% extra weapon damage and deals no damage by itself.
Grimlord, on the other hand, has a consistent 60% melee damage boost for a total of 120% melee, making it by far the best darkness guest available for FD beastmasters - but with the understanding that it only covers one element unlike the Essence.
Basically, get Essence if you're short on slots, otherwise keep the Grimlord.