“Find Arvak's skull in the Soul Cairn” is a Quest in Skyrim: Dawnguard
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.
With the skull found, return to the Soul who sent you after the skull.