Anyone know where to find his skull?

Please try "search topic'. There have been lots of answers for this.

Upon entering the Soul Cairn, go alllllllllllll the way back to where you find Serana's mother. Turn 90 degrees to the right and head off in that direction.

You'll come across an obelisk standing on four "legs". There's going to be four ghosts worshiping around a shrine with a bright purple light coming from it. As you approach, they will turn into leveled undead, and once you destroy them you can claim the horse skull on the altar. This is Arvak's skull. Once you grab it, the quest giver will appear a short distance away and take the skull. in return, he teaches you a unique Conjuration spell that you use to summon Arvak from then on.

Trust me... Arvak makes travelling in the Soul Cairn much faster.

Thank you very much

I do what I can.

Just trying to help.

I apologize for not being very helpful, badantjoka4k.

Isn't nice to know that there are some helpful people like Uriel Nocturne around when you need them?

lol

I have an almost photographic memory. It's finally come in handy for something...