I've found these in Soul Cairn and I was wondering what they do anyone know?

You take three of them into the "Reaper's Lair", place them on a pedastal, and it summons the reaper, which is a mini boss fight in the Soul Cairn. If there's anything more to it than that, then I missed it.

Where is this reaper lair? I'm finding it hard to find my way in Soul Cairn because everything looks the same.

i believe is north west of were serana's mother is imprisoned, does any one know the exact location of the shards. I have only been able to find 2

I believe the location of the lair is correct and the stones, there are 3 of them, are located at or near where you fight the bonemasters to release the mother.

you mean the keepers.--- I am going to have to revisit those places again. Thanks