Find Serana's mommy in the castle entrance. (Do not enter)
Now... go right, and hug the castle wall. It's a looong wall but keep walking alongside it. The compass should show you walking exactly East I believe.
Eventually, the wall will make a sharp turn- continue walking straight!! Pretend as if the wall is still there and keep walking STRAIGHT. It should be 50-100 meters straight ahead. You will see 4 ghosts near a large- thin Altar. They wil turn to bonemen. Show no mercy.