As soon as Zoe heard
Hades' announcement, she headed to Town Hall.
Actually, the first thing she did was let out a string of profanity that would have made Mal blush, and if Grace picked any of it up later, she would only have herself to blame. Then she took Grace, promising John they'd be safe, and headed to Town Hall. She handwavily contacted the others in charge on the way, and started working on a plan for how to take care of Hades. At the moment, the plan involved a lot of violence on her part.
[Please obey the OCD!]