So I got one of them, the one behind the Boneyard near the wall with the Souls of the Soldiers. But I'm having trouble finding the others. The issue is I looked up youtube videos, one of the videos does give me the locations, but I need more context since this guy faded out, instead fo walking from the first location to where I needed to go, he fades out and starts where he needs to start. And I need some visual references to use or else I'll be searching for hours.

I think they're random. I just found out that you can skip the whole thing, though.
Use detect dead, and you'll see skeletons buried in the bonemounds in the Reaper's Lair. Go directly above where the skeleton is buried, and try attacking it. If you connect, the boss fight begins immediately.

I do not have detect dead. I don't even know what that is.

It's like detect life. But with dead stuff. Alternatively, just start wacking at the bone piles at the back of the room. You'll hit one of them pretty quickly. Just give it a few seconds to rise out of the floor.

Honestly, it's much better than sifting through chests for randomly generated fragments.

The reaper fragments are randomly generated....really...what crap...is...that. -.-

Okay I'll find the reapers area then.

I believe so, anyway. Also, the Reaper's Lair is...North East from that one gap in the wall that splits the Soul Cairn? Directions are weird, there, so I'm not sure. T-T

Soul Cairn is hard to navigate.