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1) Necromancer - as my main hero, I always prioritize Spiritual Connection, as I believe it's a very valuable skill, especially when mana is scarce. And if the game isn't against other players and there's enough time, you can use this skill to restore mana by attacking neutral creatures. Moreover, a big advantage is that after using it, the hero is stunned for only half a turn, so you don't have to wait long for their next turn.
Banshee's Cry as a separate skill is absolutely useless; it can only be applied if you have additional skills and artifacts that reduce morale and luck. In all other cases, spending an entire hero's turn on it is a waste of time. However, for heroes that you don't develop much but are likely to participate in some battles, this skill is more useful than Spiritual Connection, as it provides much more benefit.
2) Dark - first, Pain Lord. Due to Necromancers having a large amount of magic power, mass debuff is very effective (and if you get Emerald Boots, the damage will be devastating).
Next, I take Mind Lord and Curse Lord, but sometimes you need to choose which one to take first. In the case of Ravena, it's better to take Curse Lord first. In all other cases, I choose based on the enemies and what drops first. If the enemy plays with ranged attacks, it's much better to take Mind Lord (and the entire enemy build is ruined due to mass confusion, of course, if they don't have mass dispel).
Besides the Lords, I NEVER take anything else. Cursed Earth is a waste; the damage isn't worth it. Dark Seal is rarely playable, although in some cases it can be used, for example, when fighting another Necromancer; in this case, it should be taken instead of Pain Lord (because mass plague doesn't affect undead). Dark Weakness isn't that useful, and Dark Regeneration never helped me; I only took it to get Horror Scream.
3) Summoning - it's very useful to take it for raising the dead and summoning Phoenixes. First, I take Life Lord, then Magic Lord, and then I don't develop this branch at all.
4) Defense - you definitely need to take Resistance, as it's the most useful and readily available skill. And due to the large number of skeletons, it has a noticeable effect. There are the following options:
a) Take Reflection + Grave Cold.
b) Take Reflection + Stone Skin.
c) Take Solid Defense + Readiness.
d) Take Dodge + Fight to the Last.
I rarely use the first two options, although Reflection is generally useful against Chaos, and Grave Cold isn't that bad either. The last two options seem most effective to me; the third is better against Dark Elves, Dwarves, and Demons, and the fourth is better against Barbarians and especially High Elves. It's especially important to take Fight to the Last for those who have Life Lord, as it will allow you to resurrect for a couple of turns.
Defense + Summoning + Dark is my favorite combination, as you already know.
I debuff all enemies with mass plague (if there is Subjugation or Berserk, I apply them first), and then the main thing is to hold the line; defense helps to withstand the attack, and summoning magic allows you to constantly raise the fallen.
All other skills are taken as desired:
1) Education - I usually take it; in the early stages, it compensates for the lack of mana due to magic attraction, and in the late game, it gives a significant bonus to luck.
2) Sorcery - I only take it if I'm not playing as Dark, which happens quite rarely. With mass spells, this branch is simply useless. If I mainly play with summoning, this branch turns out to be quite useful. Wisdom is not a very useful skill, as a couple of spells from other schools at the same level almost never make a difference. Secret Knowledge is only useful while there are mana problems; Education + Spiritual Connection compensate for these problems, but it can be taken to get undead protection from magic, which turns out to be quite useful against mages, especially if you add Reflection from the Defense branch.
3) Chaos - I rarely take it, but it's a matter of principle - if I want to play as Chaos, I'll take Academy or Shadow League. I can't say much about this branch; it might be useful, but I don't use it. However, if you take Ravena, you can take Cold Lord + Deadly Cold; the damage will be significant.
4) Attack - not the most useful skill branch, as Necromancers don't have the highest attack stats, but in some cases, it can be quite useful, for example, if you take Lucretia with her vampires, the damage will be good. You can also take Ranged Attack, then the damage from skeleton archers will increase significantly. You definitely need to take Battle Fury for the same skeletons. Then you can take Cold Steel, but I don't take this skill, as it's not cumulative with attack skills (if it were cumulative, the overall benefit from this branch would be +40% to melee damage, and as it is, it's only 33% + this skill deals water magic damage, which means it's useless if there is immunity), you can take Tactics, this is convenient for cases when you need to strike first, and considering the undead's initiative, this is simply impossible (except for vampires, but sending them alone against the enemy is just stupid, except for some special situations), there are Fire Arrows (if there is Machine Management, this skill is a must) and Haste.
5) Leadership - forget it; even the usefulness of the skills cannot compensate for the fact that we simply lose by taking this leadership instead of some other skill branch.
6) Logistics is important only on large maps, but in this case, you should definitely not take Scouting and everything that follows from it, as you can scout the map with other heroes, and there will be no benefit from these skills later. But Pathfinding + Marching Speed + Warpath is a very useful set.
7) Luck is very useful if you combine it with Attack; on its own, this branch seems not very useful to me. On its own, Deadly Luck is a waste of a level and a skill slot, but if you combine it with Banshee's Cry, or better yet, Horror Scream + some artifacts, you get a quite noticeable effect (once I took Horror Scream, the Necromancer set, Deadly Luck, used it all in battle, and then added Grief from the hero - the result was devastating, -11 morale XD). Soldier's Luck is useless, as it affects the probability of the Prince's stun working (it seems to work often enough anyway) and the skeleton's stun (and I don't use it at all). Magic Resistance is a good thing; although the probability is small, if it works... You should add Luck in Battle and Loot to it, which will give a good effect. If there is the Tear of Asch on the map, you can take it instead of some other skill; then you can find the obelisk even without visiting all the obelisks.
8) Machine Management - it's purely a matter of taste; the branch is effective approximately as much as it is effective for other factions. Although for Kaspar, this branch is very effective - the tent will heal your troops and kill enemy troops.
