You did everything RIGHT. The first important thing is every boss drops an item.... for Eikthyr it is a couple "Hard Antlers". With those you build the Pickaxe to destroy Stones.

His Head you hang up, and then you can go there and push "E" on the head to gain his "Forsaken Power". Every Boss has one Forsaken Power. You can use it once every 20 minutes and it lasts 5 minutes... they are all the same in duration and cooldown.