I had built up a stock pile of over 1,000 mana. I was going to nuke 2 of the remaining 4 enemy cities with Curgen's volcano.

That is when the clever ai was able to take one of my cities defended by a champion (not my sovereign). I lost the battle, and then had ~500 mana.

Then the ai attacked the city my champion (not my sovereign) fled to, killed him again, and then I had ~250 mana.

Then they did it again! And I was down to ~125 mana. Under some conditions, losing a city and/or losing a battle defending a city with a champion causes half mana loss. Maybe all champion flees halve your mana store, in which case I obviously don't lose champions enough and don't stockpile mana very often!

I understand that when your sovereign dies he flees and you lose half your mana. I've played ~90 hours and never noticed that it also happens if one of your champions dies.

In case this is a bug, and it only happens sometimes, I had cast paragon twice on the champion (Argus I think his name was). And the other freaky part was this chain of events:

1. Attack city

2. Win battle, but argus gets wounded, 2 of my mundane units survive battle

3. Instead of occupying the new city, argus and the 2 mundanes appeared in the nearest city (is this a bug?)

4. A copy of Argus also appeared in another city!! (definitely a bug!!)

Seriously, I've played a lot of this game and this was the weirdest scenario I've had.