Well, the Wayfinder is mentioned by name in the final line. Originally, the third line described Kef Bir (“Beneath the depths of misty fells”), but we decided to change it because the dagger is supposed to point the way to the Wayfinder regardless of where it is, so having a description of its current physical location inscribed on the blade would contradict that.
Oh yeah, I was agreeing with your inclusion of the word ‘Wayfinder’, since others were suggesting it shouldn’t be a part of it.
Here’s my shot at what the inscription could be:
This blade only whispers to a darkened heart
It will guide to the place where the Wayfinder dwells
Reveal your hatred, reveal your might
To those that it chooses, only this blade tells
The phrase “only this blade tells” refers to the whispers in the third person (instead of “only we tell”), so maybe the rest of the inscription should too. For the second line I think the dagger’s new role should be abundantly clear, for both Rey’s new vision guiding her to Endor, and her following the whispers on the DSII wreckage. The third line implies an active roll: you must tap into the dark side to “activate” the dagger, so again, Rey’s descent to the dark would be more clear. And I like the idea of the dagger already “choosing” and “calling out” to Rey from the moment she picks it up, meaning it can sense there’s already enough darkness within her. She’s already been tapping into her dark side since the opening (and the previous movies), and we even see her cast lightning moments after finding the dagger.
I also tried to make it sound a bit more ominous and evil, and it also sounds like a twisted version of TFA, when a different blade whispers to Rey, and Maz tells her to embrace the light, that it would guide her. It might subtly help make that parallel I mentioned earlier more apparent.