Imps are sitting in a dungeon core. Mana for imps blocked. Imps a rest.
Windows 10 64 bit, 8 Gm RAM, Intel(R) Core(TM) I7-4500 CPU1.80 GHz 2.40GHz GPU: AMD Radeon R7 M265, Intel(R) HD Graphics Family. Version game fresh, 1.5.2f4
In the long game, the imps fall into the dungeon core. They remain there. AI can't get them out (don't know). Can only person (I've experienced this in my own dungeon core). And these mana imps blocked (as usual). Ie part of imps idle.