At the start of turn 10 check the message area, you'll see something like:
Your soldiers are tiring, concentration is harder, mana and rage generation are halved.
On turn 20 you'll see something like:
The battle is long, your soldiers have lost their bloodlust, you can no longer gain mana or rage.
I'm playing a mage, but don't have the concentration skill (yet), normal difficulty, I'm level about 12, but I started noticing these messages when I started making battles last longer to exploit Sacrifice.
I regain 2 mana per turn upto turn 10, then 1 mana per turn upto turn 20.
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