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Vadimh is slightly mistaken.
Imps (an alternate upgrade for Fiends) steal twice as little mana as Devils (the standard upgrade). Therefore, if you skip a couple of turns with Imps due to mana drain, the effect will be the same as with Devils at the very beginning of the battle without skipping turns. So, if the goal is simply to deprive a Necromancer of mana, Imps are not very suitable. It is better to open a gate with Devils.
Imps (an alternate upgrade for Fiends) steal twice as little mana as Devils (the standard upgrade). Therefore, if you skip a couple of turns with Imps due to mana drain, the effect will be the same as with Devils at the very beginning of the battle without skipping turns. So, if the goal is simply to deprive a Necromancer of mana, Imps are not very suitable. It is better to open a gate with Devils.