Necromancy

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Necromancy
	Basic Necromancy: 10% of enemy creatures killed are resurrected (5% in HotA) .
	Advanced Necromancy: 20% of enemy creatures killed are resurrected (10% in HotA) .
	Expert Necromancy: 30% of enemy creatures killed are resurrected (15% in HotA) .

Necromancy is a secondary skill that enables the hero to raise killed enemy creatures as Skeletons (or Skeleton Warriors). All Death Knights and Necromancers start with basic Necromancy skill, except Isra the Death Knight and Vidomina the Necromancer, who both start with advanced Necromancy. Other hero classes cannot learn Necromancy, but they can acquire it from Scholars, Pandora's Boxes, Events or Witch Huts (unavailable by default).

In Horn of the Abyss, the skill percentages are decreased to 5%, 10% and 15%. 

Bug: AI heroes always raise exactly 1 Skeleton (or Lich), even if it should've raise 100% of the creatures slain. In Horn of the Abyss  this bug was fixed but another bug appeared: AI somehow raises more Skeletons than it should be.

Heroes with Necromancy as a starting skill:

• All Death Knights.
• All Necromancers.

Heroes with a specialty in Necromancy and starting at Advanced level:

•   Isra the Death Knight
•   Vidomina the Necromancer

Skill affecting factors:

• Amulet of the Undertaker – artifact increases Necromancy skill +5 %. (2,5 % in HotA) 
• Vampire's Cowl – artifact increases Necromancy skill +10 %. (5 % in HotA) 
• Dead Man's Boots – artifact increases Necromancy skill +15 %. (7,5 % in HotA) 
• Necromancy Amplifier – structure increases Necromancy skill +10 %. (bonus stacks if you have multiple towns with this building) (5 % in HotA) 
• Soul prison – Necropolis grail structure, which increases Necromancy skill +20 % (bonus stacks if you have multiple towns with this building via Map Editor)

Additionally, with Cloak of the Undead King it is possible to raise other undead creatures instead of Skeletons. However, the cloak does not affect the skill percentages.

How it works

All killed enemy creatures can be raised as skeletons. This includes sacrificed creatures, cloned creatures, summoned elementals and even war machines. If the victorious hero's army has no empty slots available and no Skeletons, but includes Skeleton Warriors, the killed enemy creatures will be raised as Skeleton Warriors, but only 66.6% compared to Skeletons. If there are no available slots and no skeletons (either kind) in the army, none of the killed enemy creatures are raised.

The number of skeletons brought back at the end of the combat depends on three main factors:

1. Number of creatures in killed stacks
2. Amount of health of killed stacks
3. Level of Necromancy skill

First, the total number of raised Skeletons can never exceed the number of creatures slain in combat. Second, the total health of raised Skeletons can never exceed the total health of killed creatures. Third, the level of Necromancy skill used in calculations can never be more than 100%, although it is technically possible for a hero to have Necromancy skill above 100%. These rules in Shadow of Death are applied to every killed stack individually (in Horn of the Abyss  the amount of creatures raised is calculated by the overall number of killed creatures), and always rounded towards 1.

The health of the enemy creature killed affects the efficiency that it is revived as per table:

1 hp = 16.6% efficiency
2 hp = 33.3%
3 hp = 50% efficiency
4 hp = 66.6% efficiency
5 hp = 83.3% efficiency
6 hp or higher = 100% efficiency

So a hero with 50% necromancy, who killed 100 creatures with 6 hp or higher, will obtain 50 skeletons, whereas if he were to kill 100 peasants (who have only 1 hp), only 8 skeletons would be obtained. This can be also interpreted as the hero being able to revive only 50 HP total (8 skeletons have 48 HP).

Additionally, with Cloak of the Undead King the same rules apply to Walking Dead, Wights and Liches. As they have more health, less is raised. And if a hero only has upgraded versions in the army and no free slots, the "two thirds rule" is applied (the health difference between basic and upgraded creatures does not play any role in this case).

Chance to get

Out of total 112 for Shadow of Death, and of (112 + Interference chance) for Horn of the Abyss .
Banned skills change the probability.

Town	Class	Chance to learn
Castle	Knight	0
Castle	Cleric	0
Rampart	Ranger	0
Rampart	Druid	0
Tower	Alchemist	0
Tower	Wizard	0
Inferno	Demoniac	0
Inferno	Heretic	0
Necropolis	Death Knight	10
Necropolis	Necromancer	10
Dungeon	Overlord	0
Dungeon	Warlock	0
Stronghold	Barbarian	0
Stronghold	Battle Mage	0
Fortress	Beastmaster	0
Fortress	Witch	0
Conflux	Planeswalker	0
Conflux	Elementalist	0
Cove	Captain	0
Cove	Navigator	0
Factory	Mercenary	0
Factory	Artificer	0

User commentary

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Necromancy is the basis of Necropolis faction. By default there cannot be a Necropolis hero without this skill, so if a very important basic skill like Wisdom or Earth Magic is offered together with improving Necromancy, ignore Necromancy. You will get it sooner or later, while an ability to learn Animate Dead and cast it properly won't appear for every level. However, if your hero wears Cloak of the Undead King, expert Necromancy is exactly what you need, as rising Liches, the only Necropolis shooters with good stats, will make you almost invincible. In that case you might want to avoid battles prior to getting expert Necromancy, as 2-3 level Necropolis troops are significantly weaker than many other units of those levels. For heroes from other towns, the Necromancy skill is undesirable as Skeletons are weak, slow and will spoil morale, and getting somewhat 5 Skeletons after a minor battle will be a total nonsense. Thus, if Necromancy came to a non-Necropolis hero, a better decision is to reload the game and avoid the source of it, unless your game is dedicated to Necropolis - if so, you may hire a supplying hero regardless of the class, who will increase the number of Skeletons. Necropolis and Necromancy are often considered to be overpowered, and therefore frequently banned in multiplayer games (and tournaments).

See also:

• How does the necromancy skill work exactly? Heroes Community thread started by angelito.
• Secondary skills