Necropolis is one of the main factions found in (almost?) every Might and Magic game. I am not going to cover their lore as there is enough of that around on the web elsewhere.
Necropolis does not have the strongest army as measured by individual unit strength, but they have by far the most durable – not because they have beefy units, but because they are masters of healing. Their magic attackers deal dark damage, of which they have 3. Necropolis is also the master of ranged arts as they can have three permanent ranged units.

• Skeleton / Skeletal Spearman
  The Skeletons are a basic ranged unit, that deal half damage to anything further than about 4-5 cells. They are pretty weak if it comes to damage and health. The upgraded version applies a 3x3 slow area effect, which can be useful to keep enemies at distance
  
• Ghoul / Ravenous Ghoul
  The ghoul is a sturdy and strong melee fighter. They take reduced might damage and the upgraded version deals 30% additional damage to living creatures as well as extending their speed by 2 when attacking them. Not only that, but compared to other faction's units of the same tier, the Ghouls are the most damaging creature before the 30% bonus. This makes them ideal for rushing to the front and deal devastating damage.
  
• Ghost / Specter
  Ghosts are rather fragile constructs as they have low health. However, also they get 25% reduced might damage as they are incorporeal. They deal a good amount of dark magic damage and reduce healing of any stack they attack for a few turns. The Specters gain two charges of a skill that heals an undead target by a fix amount and an additional amount based on damage dealt to surrounding units of the target stack. This basically allows a stack to be restored almost back to full health. Use this skill only if you have no other options as they have only two charges. All in all the Ghost is the Necropolis' healing unit.

• Lich / Archlich
  The most famous of the necropolis creature are the liches. They deal ranged dark damage and reduce dark resistance for the target and the 3x3 area around it. They shoot efficiently at any range and are a quite able fighter. The upgraded version adds one charge of lifedrain, which can be used on a friendly creature before the attack. Damage dealt in this turn is transferred as health to the buffed friendly unit. There seem to be restrictions though of when this actually works. Generally, apply the buff to a core creature and hit larger creatures. This seems to always work.
  
• Lamasu / Putrid Lamasu
  Lamasu (Lamasi?) are flying creatures that emit a weakening breath on their surrounding units on the start of the round. They are not exceptionally fast or strong in any other aspect, so they are best used to lower health and resistances of their enemies. They are however said to have the best moral trigger of the undead, often allowing them to attack twice a turn.
  
• Vampire / Vampire Lord
  Vampires are also very famous ones of the necropolis creatures. They have an innate 25% lifedrain attack. They can also teleport around the battlefield, making them quite mobile. Their speed however is low. The Vampire Lord has an ability to prevent taking direct damage through attacks / retaliation.

• Fate Spinner / Fate Weaver
  The Spider is the only creature who can transform itself. It starts as a melee fighter (human form) and has a skill to transform itself to a ranged fighter (spider form) and back. The spinner in human form deals damage do the first attacking creature. In spider form it deals earth damage per turn for every attacked and attacking creature. The weaver in human form can use an ability to damage all adjacent units with prime damage that can't be healed. in spider form it can cast a net on adjacent creatures which can not act and lose their turn.

In this section I am going to cover what the basic actions in a fight might be regardless of skills, hero, blood or tears and might or magic.

As masters of healing, necropolis depends only on two things: gold and the altar of eternal servitude. The latter enables the resurrection of any fallen unit at a 50% higher price. That's where the gold comes in. It is advisable to get this building somewhere in the first week if crystal reserves allow it. If you have the option to buy new creatures or resurrect old ones, always choose to resurrect, even if you get fewer units instead. Necropolis is the only faction that can play the entire map (aside from maybe the last battle per map) without any permanent unit losses. So it is imperative that you get your gold income flowing in the first week by upgrading the first 2 levels of the Town Hall and getting a marketplace.

On the battlefield necropolis excels at preventing losses due to enormous amounts of healing from units and their racial ability. Because they have good ranged fighters, I tend to send one stack to the front (usually Ravenous Ghouls) and keep them alive using all the healing abilities. I only do this because it is easier to heal up heavy losses on one stack of units than few losses distributed over many stacks. Keep Specters in the back row defending until their time to heal has come because then they'll still profit from spirit form, reducing damage dealt to them by 50%. Transform the fate weaver to its spider form and take out enemy shooters first or anything that is scary close to your shooters. Use Vampires, Lamasu and Specters to block access to your shooters for as long as possible. With Tactics 2 you could achieve something like
______________
ALL
SLL
FFS
FFG
----V

(A = Archlich, S = Skeletal Spearman, L = Lamasu, F = Fate Weaver, V = Vampire, G = Ghoul, S = Specter, ___ = Upper battleground border)

With this setup, the fourth level of the racial ability can heal 5 units at once. This is very useful if you are getting hit by a fireball or similar.

Magic

• Initial: Asha uses all (Passive)
  You get an amount of Mana for every stack destroyed during combat
  
• Blood I: Mark of the necromancer (Passive)
  Enemies are 20% more susceptible to blood abilities
  
• Blood II: Curse of the Netherworld (Active, 1 Charge)
  Deals damage across all living enemies on the battlefield and distributes half of it over all friendly units
  
• Tears I: No Rest for the Wicked (Passive)
  Increases power of necromancy (your racial spell) by a percentage
  
• Tears II: Sacrifice (Active, 1 Charge)
  For 2 turns, all damage dealt to your troops are redirected to target friendly stack.

Might

• Initial: Face of death (Passive)
  When an enemy stack dies, all remaining stacks suffer from a loss of morale, luck, damage and resistances for 1 turn
  
• Blood I: Vampiric Embrace (Passive)
  All units profit from life drain which heals them for a percentage of the damage inflicted
  
• Blood II: Doom (Active, 1 Charge)
  Dooms target unit to perish in a certain amount of turns unless it is killed first
  
• Tears I: Bound by Death (Passive)
  Your units who start their turn adjacent to corpses of killed stacks leech their departing life forces to heal themselves by a percentage of their own total health points per adjacent stack.
  
• Tears II: Death is not the End (Passive)
  When an own stack dies, they drain an amount of health points from their surrounding stacks to heal revive themselves. Only happens once per stack.

I can not give a clear answer to that question. I just can give you some thoughts on the abilities.

Necropolis heroes make excellent mages due to their ability to regenerate mana on slayed enemies. They deal 20% increased damage with their damage spells upon reaching the first blood tier and the second allows for a further devastating spell. Tears allow very good handling of losses because the first tear tier (that word...) increases the power of necromancy, while the second tear tier allows you to bundle damage dealt to all sources on one stack. Additionally, if you go tears you should consider using Mass Despair and Mass Weakness. Enemies will likely skip their turn due to bad morale and if they attack, they deal less damage.

As awesome as mages sound, the might heroes are not far less impressive. I personally think that doom does not really have good uses. It would require such a big enemy stack (or boss unit) that the large amount of turns is faster than your troops attacking it. That's why I think Blood is less advisable. If you go blood however, make great use of life drain and if possible enhance it with the mass drain life spell. Tears on the other hand allow your units to have free resurrection options. They are quite tricky to use efficiently, but if you can, they are really strong. Furthermore, the initial ability is stronger than it may look. It is rather easy usually to get rid of one enemy stack and that makes the rest of them become weaker. However, a big minus for might is that you can not use all war cries. Heroic Charge and (Mass) Heroism require the use of Morale, which is disabled for necropolis as they don't use it.



There are a few skills you should / could consider in general or specifically for Necropolis:

• Rush
  This is one of my all time favourites. Basically, whenever you get your very first action, which is hopefully before enemy ranged units had their turn, apply Rush on a unit with a good deal of speed. Not only will it be able to act next due to higher initiative, also it receives 50% speed bonus. This allows the troop to get to the enemy lines in no time, blocking ranged fighters or at least make them target him. It is far easier to handle losses of one troop than for many.
  
• Storm Arrows
  Skeletons are weak I said, but this is largely due to their range penalty. Storm arrows not only removes range penalty, it also increases damage done by a good deal, even for pure might heroes.
  
• Life Drain
  The all time good Life Drain spell. Smack that on your vampires and they get around 80% life drain with the perfect settings. (spell ~30% + troop 25% + Arachne Weapon 15% + Blood Tier I ~10%)
  PS: Sucks against necropolis
  
• Cleave
  This passive is awesome. The first friendly unit to kill an enemy gets an instant free 2nd action. Whether you rush the enemy with a melee stack or a ranged devastating attack strikes, you'll attack a second time with that stack. It is advisable to take out one enemy stack first, even if it is no high priority target if you can not kill another stack. As necropolis might hero, this already applies the Face of Death effect.

Arachne[mightandmagicheroesvi.wikia.com]: Features lowering enemy initiative, morale and a 15% life drain to your own stacks
Edge of Balance[mightandmagicheroesvi.wikia.com]: Skills use the higher value for blood or tear for any skill for the first 4 turns as well as enables the hero to use abilities of blood and tear for the first 4 turns.
Dragonbone Staff[mightandmagicheroesvi.wikia.com]: Lowers enemy morale and enables summoning of up to 8 spectral dragons.
Soulreaver Staff[mightandmagicheroesvi.wikia.com]: Boosts various aspects of dark magic.
Staff of Sandro[mightandmagicheroesvi.wikia.com]: Features many enemy debuffs and a charge to make hero attacks on enemies heal a targeted friendly unit

The undead shine in every way you play them. Although they have weak units compared to other factions, they shine in crippling the enemies with powerful debuffs, outnumbering them in the long run and have strong healing capabilities. I am still trying to get my Arachne to level 4, so I can try to play with a blood might hero carrying Arachne, cast Drain Life and attack with vampires for ~85% life drain.