Ras is the dancing longsword bought in Trademeet. How do you get it to "dance" and fight on it's own?

I cant confirm this, but I believe Ras' 'Dancing' ability is activated via the 'items' Icon on the character tool bar (The icons at the bottom of the screen when a single character is selected.)

Look for a backpack.

Yeah, its by using the Items button can you select the blade's ability.
But, Really, It doesn't help much, IMHO.
Ras isn't like a Black Blade of Disaster or a phantom blade, but more of a flying sword that barely hurts a vampire, if it manages to hit it in the first place!

I say, that you use it only if you need that little extra help. But don't count on it staying there to help you long enough!