is there suppose be something important in here?

You can summon an optional boss to fight by placing 3 reaper shards on the pedestals.



Though if you're like me you have the three shards, but can't find the reaper lair.

Where is it located in the Soul Cairn ? Close to the castle ?

Go to the castle, and while facing the castle turn right. Follow the wall that way, and you should run into it (it isn't ON the wall, so check any buildings you pass. The door will be on the opposite side of the building from you if you're following right beside the wall).

all u get is a handfull of black soul gems and dun dun dun dun... an iron battle axe. have fun