Shadow Rift doesn't work as described

I got shadow rift through a quest and held off on using it until near the end game. 

A drake was threatening a city, so I cast shadow rift, the demons appear and just stood around.

Walked over and occupied the same tile as the drake, but did nothing.

Later an enemy AI attacked, so I cast the spell again. Still nothing. the demons just let the AI's army pass by without a scratch.

Per the spell description, the demons are supposed to attack anything in their path. however that's not happening.