So I've been playing a goolock this patch, and while I like the idea of this new feature(even though it's straight home brew) I don't feel it procs enough to be comparable to the Fiend's Dark Ones blessing.

Dark ones blessing is very easy to get up, and it gets better as your level goes up.
Mortal Reminder requires a critical hit to proc. Critical hits just don't happen all that much in D&D, and right now at the levels we're playing at, if we do get a critical hit, it's usually also a kill.

I think a better way to make mortal Reminder feel more useful is for it to proc on any hit, not just critical, but to balance that out, don't make it an automatic frightened. Give it a wisdom saving throw so the enemy has a chance to avoid it.
Mortal Reminder would now be avoidable, but it would proc alot more often making it more useful in the long run.
Right now, it just doesn't really do anything, due to only activating on critical hits.