I wouldn't know, but…

(click to show/hide)

… what I've planned for the ending required Serenity wearing a certain outfit when going to the final boss's room. She then becomes evil and joins the Castle Master as an ally. With Tranquility, a clone of hers, treating Serenity as a sister.

When Newerth ends, Serenity and Tranquility and the Castle Master and everyone else in the castle would survive, and thus the Castle Master will have taken over the world… or what remains of it, anyway.

Avoid that spoiler if you don't want to be spoiled with what I'd think constitutes as a "worst ending".