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Yes, the demonologist is a classic – he is undoubtedly strong, but it’s complex to develop him to a decent level.
The ninja is quite useful, but sometimes it’s not possible to fully develop him to his maximum potential. Death has another excellent class, the assassin, which is the best choice when developing a death mage, especially if, for example, the rogue has gone into a ranger or a field marshal (a wonderful class, by the way), or if there are two necromancers. To go first and also have a necromancer is quite a significant advantage, for example, against life – it’s a sad situation, and the battle will take place under the control of death, or at least the first round, which is sometimes enough to secure victory in the battle.
In addition, the assassin will not be as weak as the demonologist or the ninja, and it will be difficult for the opponent to kill him with a single blow.