Playing during the UG campaign, the level where you have heros attacking your crystal every so often, and the hero fortress in the middle. It seems that when the other underlord kills heroes, your imps will try to banzai through the fortress to pick them up even though they will die in the process. This was happening even when I did not have vision of the other side. Impasse flags did not stop the imps running through, so I had my entire collection of imps trying to suicide themselves every wave. The only way to fix was to sell my prison.
This didn't seem helpful behavior, especially when I didn't have vision. I'd suggest imps be changed to ignore heroes that another player KOs if possible.
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