Eviln spell

[Hento]

What's up with this spell?
I tried to use it but it said no units to use spell on, and the variety of opposing army was pretty substantial, so if it doesn't work in most cases what's the point?

It' s good I quicksaved before buying it, so I didn't buy it in the end(it's very expensive).

And what does kill squad mean? what squads?