anyone know how to get this thing? thanxs

Beginning of Dark Brotherhood

I presume you can also get it by killing the executioner when you first arive in solitude.

Wait which hood? The one the executioner wears or the one being executed?

If the former then go by what tyraz said

The one in Solitude that performs the execution is a named one and doesn't have a mask.Or is there another one?

The Dark Brotherhood initiation sees you in a locked room with three targets. All three have execution hoods - whichever one you kill (if not all of them) will drop an execution hood.