Age II hero energy
What causes a hero's energy to drop all of the sudden, and what makes it come back up...besides buying potions?
I believe it drops every time you send the hero on a mission. I noticed yesterday my attack hero was down to 94 energy after taking valleys for me, and they were low level valleys because I just started playing 3 days earlier.
However, they do recover their energy over time, so if you have one that is running low and don't want to pay for something to restore it, just let that one take a break and use another hero.