Trying to collate information about this and see if we can help the devs track down what the cause it.
Currently I’ve had this happen in a team the Stone Wall challenge. 3 man team. Hero (Daemonomnicon), snowy owl, giant spider.
Had reports of Abnyssia not summoning either.
Anyone else got occurrences?
EDIT: @yonizaf suggesting that it’s due to starting with a team of three or less troops. Start with four and kill one and you should be fine.
Yes. Happened to me too. I tried only using Abynissia and Princess Elspeth (2 slots empty). I casted Aby like 5 times and she didn’t summon Infernal King.
Here’s a link to my post (with screenie) of Abynissia not working: Hello Ladies!
Yes I also had same hunch as @yonizaf that it’s starting with the empty slot causing the issue. I’d be happy to see this fixed though, I don’t want to have to kill something before I summon or this is going to be so slow it’s not worth doing…
Black Beast or Sacrifice would be the way, but then need to cast the summon, and then somehow win… yawn…
Someone suggested Sacrificial Priest, he has a chance to summon Abhorath (which is also a daemon).
Tried again with 4x Sacrificial Priest and it worked. Thank @yonizaf
So we can’t “cheat” by empty some spots in our team intentionally, lol.
Unfortunately this bug progressed this week event from bad to worse 
.
They’ve said they will be lowering the requirements for this weeks event.
Thanks 
It’s the worst event so far anyway - even if the top reward was for 50 stones, which I don’t think possible. At least for me, but I guess many will share the sentiment…
Getting two stones per battle would still mean 200 battles minimum for the top Event reward. Ugh.
Edit: just noticed it was said they’re going to lower the reqs. Yay!
It will be less than 400 stones.