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There are many options.
1) Kaspar with machines and summoning - upgrade archers and zombies in the first week, take a ballista and a cart (it's better to have other heroes deliver all these goodies so as not to waste time) - and then just go to the enemy's castle, destroying everything that moves along the way, and upgrading liches, archers, and zombies. With a tent, ballista, and expert summoning, there is almost no chance of countering the raising of the dead by the end of the second week. Ideally, there should be 2-4 stacks of 1 bone warrior each - to protect the archers, zombies, and liches from arrows, at least for the first time.
2) Summoning is good not only for Kaspar in the early weeks - Lucretia, Vlad, Naadir, Orson, and especially Zoltan (due to the large amount of mana that accumulates when leveling up education) also perform very well. An interesting option is to level up fire warriors - in rush games with early battles, 20-40 fire elementals play a crucial role.
3) Lucretia with 40+ vampire princes (upgraded in the castle), ghosts (better as spirits), and dragons (if the game lasts long enough to build them). With Lucretia, we level up logistics with Death's March, summoning, divide the vampires into 2-3 squads - and go to the enemy castle - it can be taken without any problems.
4) The tactics for long games are described by me in the thread about undead and dwarves, in the same section.
Useful things for the undead:
1) Spiritual Bond - allows you to conserve mana - it's easy to gain an advantage over squads with zombies, demons, etc.
2) Eternal Slavery - will help keep squads intact without additional use of the "Raise Dead" spell.
3) Piercing through almost any creature due to the first two points. Where other races stop in the early stages, the necromancers pass without losses (for example, cave lords).
4) Early capture of Utopia - as a rule, it can be guaranteed to be taken without losses as early as the 3rd week. However, it is not uncommon to capture Utopia in the 2nd week with relatively small losses. There are all sorts of tactics:
a) Lucretia with one squad of vampires (there should be at least 30-40 of them - only they should be raised), 6 stacks of 1 ghost each, and expert summoning.
b) Orson with Battle to the Last and either summoning or battle frenzy - place archers in the corner, protect them with stacks of 2 ghosts (or, in extreme cases, zombies) so that the dragon's attack does not hit the archers. In other words, so that the dragons cannot even hit 2 squads with one attack.
c) Almost any necromancer above level 15 with tactics and 200-300 archers.
5) To gather the maximum army, the main thing is:
a) Level up the necromancy skill as quickly as possible to expert.
b) Raise unupgraded creatures, and upgrade them in the castle - give them to secondary heroes to level up in the castle - so as not to waste time. We raise upgraded creatures only when there is nothing else to spend dark energy on by the end of the week.
c) Build a bone pillar on the first week, giving +150 dark energy points, even at the cost of a town hall, fort, or weekly vampire growth.
Next are very effective, but not always feasible methods.
d) Get the necromancer amulet, the sooner the better, even spending money on it and buying it from the cart with artifacts in the 1st week (for example). Then raising one unupgraded skeleton will cost 1 point - then a great many of them will be gathered by the final battle.
e) Level up the main hero's mentorship and hire up to 7 necromancer heroes - and level up their necromancy. By the end of a long game, you can achieve a figure of 1000 dark energy points per week with one castle.
f) In addition to d), get as many amulets of mastery as possible (this usually works well on random map games) and put them on secondary heroes.
g) One of these 7 heroes can level up "Habitable Mines" with the help of mentorship.
h) Hire 2 any heroes at the beginning of each week, take their troops, convert them into undead, and fire these heroes by the end of the same week.
i) It is better to capture 1-2-3 or more any cities as soon as possible and fully develop them - then the main feature will start to work - the undead castle converter.