Uninvited guests.

Also, the description promised chain lightning; in my case, it was just regular lightning that struck all enemy troops at the beginning of the battle with a 100% chance.

In this context, chain lightning = a chain of lightning. If you use chain lightning as a spell, it can hit your own units. This is more likely to harm than help. ;)

Added 6 minutes later
I've uploaded a new version 2.0.0 (I really hope this is the last one). Changes:

1) Urkrass's specialization has been changed. Now, instead of inspiring troops during the battle, he inspires 2 to 7 units at the beginning, depending on his level.
2) Elva's specialization has been changed. Now, she only attacks at the beginning of the battle, hitting 2 to 7 enemy units, depending on her level. Her magic power increases by 1 for every 3 levels, instead of 4.
3) An error with level 1 dwellings has been fixed.
4) The AI's development speed has been slowed down even more.