I don't know if this has been brought before in the lore forums, but while stooping around Volkihar Castle, I stumbled upon a Mythic Dawn tapestry in Harkon's quarters...
Now how did that got there? Have the Volkihar participated in the assassination of Uriel VII? Do they have ties with Dagon? If yes, how does that reflect on their relationship with Molag Bal?
Any suggestions explaining this to some extent would be welcome!^^

Someone screwed up most likely

Maybe Harkon was involved in the Oblivion crisis? But that deos not make sense either since the vampire count in Skingrad says that Mehrunes Dagon has no more love for his kind then mortals.

Maybe Harkon just came across it and liked the look of it.

Harkon was a power hungry fool. That might have something to do with the banner. I don't know what, though...

It's propably just a souvenir. He also has a werewolf pelt in his room even though he is not a werewolf.

wouldn't it be weird for a werewolf to skin himself of his own pelt though?

Most likely somebody dun goof'd.

If Count Hassildor is a reliable source, Vampires and Dagon aren't usually best pals.