It's not like it'd be hard to put in. Just add another ingredient drop to a spawn in the Abyss. Like, I dunno, Fey Wings or something. Hey, wait a minute, that sounds familiar... Those do drop in the game! And yet, they have no use whatsoever. Was Fey Slayer intended to be imbueable at one point with that ingredient, but they took it out of the imbuing menu and left the ingredient in? Someone forgot to put it in the menu? Or what? Also, there's an essence from one of the mini-spawns in the Abyss that has use as well. I think it's red? I don't remember the name.