So while HoS has yet to announce the return of ZCU15 for some mind boggling reason, I decided to do a little digging on this whole "Evil Encore" theme since not much is being said about it. Turns out not much is being said about on Tampa's side since Busch Garden's Williamsburg tells quit a bit more about this theme.
Apparently it revolves around a dollmaker named Mr. Karver (Who looks like a take on Pinocchio's Gepetto), who really brought life into the dolls he made. BGW, talks about the various dolls and their personalities.
Mr. Karver's Workshop | Howl-O-Scream | Busch Gardens Williamsburg
Though even with that info, BGW's HoS looks to have no house or scare zone dedicated to the theme. Which makes me question why bother with info of the theme if you're not really going to use it?
Then there's Seaworld San Antonio's HoS, where they're not only using the Evil Encore theme as well, but they're having a house AND and a scare zone dedicated to the theme. An' the house gives the full background to the theme.
Karver's Kradle | Howl-O-Scream at SeaWorld San Antonio
I think what happened here is the whole Evil Encore theme was created by the San Antonio crew, since they were able to make both a house and scare zone for it. For the other two parks it was dropped on them as a shared theme, so while it'll be used for the website and marketing materials, they're not gonna' use it much inside the event.