No magic when army out of control ?
 

During my latest fight against K'tahu, I ended up with one stack of paladins left who got bitten by a tirex and subsequently got a fear-effect on it.

From then on the paladin (and consequently, the entire army) was out of my control. Unfortunately, I couldn't cast spells anymore either. Otherwise I could have dispelled the fear, or use a geyser attack or similar to deplete the surrounding armies. Now, my paladin simply received repeated tirex attacks and remained "feared". And so I could only sit and watch, with over a hundred mana and several spells to help him, but not allowed to use any.
This doesn't seem normal to me.

To be able to cast spells you have to have control over at least 1 stack.

You can use this also to your benefit against heroes. They can't cast any spells if they have no control over their stacks.