I am playing Dawnguard, the Skyrim DLC, and I have been given a quest to find Arvak's skull in the Soul Cairn. It is a big place, so can anyone lead me right direction?
After you pass through the "gate" where you met the questgiver, go EEEN (about 70 degrees right from N), it's a long walk but eventually you will find a shrine-like structure which is basically a roof supported by 4 columns, it will be illuminated from within in purple, the skull will be at the foot of a pedestal at the shrine's center. The shrine is due east of the entrance to Valerica's prison.
Here's a video guide.
I find the Soul Cairn to be hard to navigate (which is very unusual for me). Typically, I don't have a problem with finding Arvak... eventually.
There is, however, a mod which is available on Bethesda Net (but not on Nexus, oddly) which provides quest markers for both Arvak and the Opus.