How does leadership become a key to the Necromancer's success?

It's very simple – it's all due to the combination of Death Herald and Diplomacy skills. This way, you can recruit all sorts of neutral creatures, and your army grows rapidly. Plus, with leadership, you can add good level 7 creatures to your army without morale penalties, instead of those miserable bone dragons. Plus, elementals and phoenixes no longer fight each other when you summon them. Plus, there's a path to the Abandoned Mines, where you can easily get 2k ghosts in the late game, and with the Blood Stone and Luck, the ghosts deal so much damage that it's amazing.

Added 5 minutes later

The Knight takes down the vampires first.)))

It's very important for the Knight to have teleport! Act only on attack. Throw all the fighters into the thick of the enemy (otherwise, the Necromancer will outnumber them, and they will kill each other). Of course, ideally, you should have the Shield of the Gnomish King.

The Knight can take down the vampires first, especially if they are paladins and not Isabel's knights. Teleport is a good thing, but you still need to find it, and where will you get antimagic then? In my opinion, the most effective spell is not berserk, but charm or berserk in combination with griffins and zealots. If you take Keen Mind, I think you can seriously damage Haven already in the early stages. It all depends on the balance of power and the artifacts; Haven is traditionally considered a cheater, but it has weaknesses