There is a bug, when you summon Magtheridon, and the enemy has 5 or 6 minions. Only 1 or 2 is summoned and there fore never dies so Magtheridon spawns.
That’s not a bug, that’s a missplay.
Or do you mean Magtheridon will awaken when enemy board is full?
Well happens to me right now. Board of enemy was full and supposed to destroy the extra minions of Magtheridon. Also, it was stuck for a second time when I used flamestrike and destroy it all of them, card does not trigger. l will tell you more as I craft the card and I have already include him on my deck build.
Greetings!
Thank you for your reports. Currently, we are not aware of any issues with Magtheridon minion card. If you think there might be a bug in-game mechanics, please submit a bug report for our developers on the Bug Report Forums.
Thank you 