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 drain the mana of a Necromancer, Imps are not very suitable. It's better to open a gate with Devils.

The whole point is that in a long battle against Necropolis, after 2.5 turns, black Devils will drain more mana than red ones. At the same time, while they are alive, the Necromancer will not be able to drain mana using Mark. The Necromancer has nothing to quickly kill the Devils with. Yes, he can cast Summon, Berserk, or Blind. But the fact that he will use high-level spells on the Devils means that the main combat stacks, such as Knights or Demons, will be fighting. And the hero might also have Vampirism.