What you were expecting to happen, and what actually happened:
I got a valraven on the fight with Ysabelle. The sigils it had would have allowed me to finish the class trial completely, without having to buy more. Since I used Darkstone troops, primarily Magnus and Essence of Evil weapon, the Valraven was inflicted with terror and ran away. I did not receive the sigils for killing it.
How often does this happen? When did it begin happening?
Once (so far), plaguelord trial.
Steps to make it happen again
Valraven fleeing due to terror instead of its spell cast.