What you were expecting to happen, and what actually happened?
Repeated challange battles in Karakoth (Dark Magic) showed me few times that Daemonc pact from Summoner worked even that it is locked. I run battles in Warlord 2 difficulty level.
What are the steps to make it happen again?
Run few battles to enable proc of the Deamonc pact.
Do you have any screenshots or video you want to share with us so we can see the problem? Attach them to your post!
See the summoned Ancient horror in the first possition.