"Paratrooping" enemy armies

[Arryn]

I'm playing Jotunheim (Utgard) on Orania (a lovely and large map with room to grow) and it's turn 75ish. Pythium (difficult AI) keeps popping 30-40 strength armies of madmen/flagellants deep into my territory, one such army 3 provinces from my capital. The AI has been popping 2-3 armies a turn into my "rear" areas for over a half-dozen turns now. While they are easy to crush with my giants, the AI is building and dropping them on me faster than I can build new armies to chase them down, or move my frontline troops (which are busy crushing Pythium's border temples) back to deal with them.

My (newbie) question is this: what magic (spell, presumably) is the Pythium AI using to send these annoying armies deep into my lands? I sure would like to do unto it what it is doing to unto me. I have stronger armies, and I'm pissed and looking for payback ...