It seems like every time you turn around there is a new Overlords event. This will be the 11th version. At this time I don't know if it will be energy or stamina based but we are reverting back to the bosses used the last time it was stamina. There is a good chance the energy based fight didn't work out for Zynga or they finally want to mix things up. If you have a balanced account and use a script, it really doesn't matter. The loot rewards and new ammo items are shown below.