“Find Arvak's skull in the Soul Cairn” is a Quest in Skyrim: Dawnguard
Locations
Walkthrough
Head into the Soul Cairn and search for the Soul. One of them is friendly and will not attack you. Instead, he will be calling for his Arvak.
Make your way to the East. Eventually, after a long distance, you will find a tower with a number of Bonemen protecting the skull.
In the tower you will find a black horse skull defended by the Bonemen and Mistmen. On the column in the middle of the tower is Arvak's Skull
With the skull found, return to the Soul who sent you after the skull.
Talking with him this time will have him teach you the Spell: Summon Arvak. This will let you summon the Arvak.