To answer your question, it's part of a final mission in the Croatoa Task Force, where you have to defeat Mary Macomber, the leader of the Cabal of Witches, ten times. When you recieve the Ten Times the Victor Badge, the Witch's Hat will be available for you to use in Icon. It is earnt like the Cape and the Aura.
There is currently no way to get this badge or this costume item in City of Villains. And it is only available on the character with which you get the badge on. You cannot start a new character with a Witch Hat. Only alter your current one.
However, I would like to point a couple of things out to you.
First, this was the wrong forum to ask your question. There is a "Players' Questions" forum for that purpose.
Second, you could utilize the search feature. I'm not sure if there are posts going that far back, but surely there has to be something somewhere in the Player Guides.
Actually it's in the first mission.
And doing that mission is some of the most fun I've ever had-
Two controllers (I was one of them) 5 Blasters, 1 Warshade.
Fighting ambush spawns back to back with an AV and multiple bosses in each one- dear lord.
I need another fix like that.
Can anyone tell me how to get a witches haton CoH, and is there a way to get one on CoV??