In arena you can get an item called a 'Mark', which gives you spells you can cast, etc etc...

what is a mark though? is it like a tattoo or what?

They're in Daggerfall, too. Nobody knows what they are supposed to be.
I used to think they were armbands that go over your bicep or something, but I don't know.
They just look like a square or diamond-shaped... something.

Could they be magical, by any chance?

I pictured a mark as a piece of paper or parchment with a magical sign on it, like a scroll.

A magical fireball tattoo would be awesome, though.

I think of them as smooth inscribed stones. But nobody really knows. I kinda like that.