Arvak

Arvak is an undead horse that appears in The Elder Scrolls V: Dawnguard.

Interactions
The Dragonborn can obtain Arvak as a mount upon completion of a side quest in Soul Cairn.

After getting the "Elder Scroll (Blood)," speaking to Durnehviir and walking to the portal back to Castle Volkihar, if the player charcter stays on the path while walking there, the Dragonborn will come across a frantic Soul calling, "Arvak, Arvak!". He will tell the dragonborn how he and his loyal Arvak came to this realm and were attacked by monsters. He had told Arvak to run, and that Arvak is still running, and has been for a long time. He begs the dragonborn to find Arvak and disappears. The player character must locate Arvak's skull in the Soul Cairn to free him. Arvak's skull can be found due east from the front gate of the Boneyard (where Valerica is found). It will be sitting on a pedestal surrounded by ghosts that turn into Mistmen. Defeat them or just grab the skull. After obtaining the skull, the soul who was looking for the horse will appear nearby and he will teach the player character how to summon Arvak.

Trivia

 * Summon Arvak is considered an apprentice-level conjuration spell, rather than an ability.
 * In Norse mythology, Arvak in one of the horses to pull Sol's chariot across the sky, the other horse being named Alsvid.
 * Arvak can prove extremely useful if the player is over encumbered and is far away from any locations where items can be relieved.
 * Arvak can be summoned outside of Skyrim's wildlands and can be summoned in certain cities like Dawnstar and Winterhold.
 * If you have the Conjuration perk "Twin Souls" you can actually summon TWO seperate incarnations of Arvak.
 * When Arvak is claimed, Shadowmere may stop following the player.
 * When riding Arvak, it is impossible for the spell to run out, meaning that the spell will start to loop and the 'active effects' tab counts down from 5 seconds over and over, until you dismount Arvak, when the spell's effect will count down normally.