Originally posted by Delle(DK):

Remember to return the first boss head to the altar where you start els im not 100% sure if you can advance.

You can. I've found the Elder's location stone before fighting Eikthyr multiple times.

Also, since no one's commented it yet: you can also find the stones showing the Elder's location at those ruined towers (the ones with multiple floors, not the tiny ones) that Black Forests are full of.
Still rare, though.