Hang trophy on the hook where you spawned.

when u first start the game there r runestones each one is a boss hand the eik trophy on his runestone u get his power

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.

Hang his head on the first of five Ritual Stones (Standing Stones) NOT the altar where you killed Eiktheir.