Undead are immune to bless, curse and blind spells, mind affecting spells and are always at neutral morale.
Living creatures dislike undead, therefore their morale decreases by 1 if there are some undead in your army.

SkeletonHealth: 6Attack: 5Shots: noneMovement: ground
60 gold (12)Hex Size: 1Defense: 4Dmg: 1-3Spd: 4 (extra slow)
Average lvl1 foot soldier, but always in huge numbers thanks to necromancy skill and skeleton transformer.
Skeleton Warrior
Skeleton WarriorHealth: 6Attack: 6Shots: noneMovement: ground
70 gold (12)Hex Size: 1Defense: 6Dmg: 1-3Spd: 5 (slow)
Numerous skeletons become even better, but running back to town and upgrading is a problem... If there is no room in your army for ordinary skeletons, necromancy skill will resurrect skeleton warriors, but there will be less of them than normal skeletons, so it might be a good idea not to upgrade cursed temple at all.
Walking Dead
Walking DeadHealth: 15Attack: 5Shots: noneMovement: ground
100 gold (8)Hex Size: 1Defense: 5Dmg: 2-3Spd: 3 (very slow)
Basically its the same skeleton with more hit points. I prefer buying 2 skeletons instead.
ZombieHealth: 20Attack: 5Shots: noneMovement: ground
125 gold (8)Hex Size: 1Defense: 5Dmg: 2-3Spd: 4 (extra slow)
Special: 20% chance to disease enemies (-2Att -2Def for 3 rounds)
Attack ratings are way too low... In my opinion, necropolis has the worst lvl2 creature.
WightHealth: 18Attack: 7Shots: noneMovement: flying
200 gold (7)Hex Size: 1Defense: 7Dmg: 3-5Spd: 5 (slow)
Special: top wight of the stack regenerates all lost damage in the beginning of each round
Regenerating ability is really good when fighting weak enemies, especially shooters.
WraithHealth: 18Attack: 7Shots: noneMovement: flying
230 gold (7)Hex Size: 1Defense: 7Dmg: 3-5Spd: 7 (extra swift)
Special: regenerates ; drains 2 points of mana from enemy hero every turn
Will get to those shooters even faster and will really annoy spellcasting heroes by slowly draining their mana!
VampireHealth: 30Attack: 10Shots: noneMovement: flying
360 gold (4)Hex Size: 1Defense: 9Dmg: 5-8Spd: 6 (swift)
Special: no enemy retaliation.
NOTHING compared to their upgraded brothers. Keep the population growing and recruit after the upgrade.
Vampire Lord
Vampire LordHealth: 40Attack: 10Shots: noneMovement: flying
500 gold (4)Hex Size: 1Defense: 10Dmg: 5-8Spd: 9 (ultra swift)
Special: no enemy retaliation ; resurrects members of their own stack by restoring health equal to the amount of damage they do to living enemies.
My favorite necropolis unit. Use them as main striking unit and you might end up with no losses!
LichHealth: 30Attack: 13Shots: 12Movement: ground
550 gold (3)Hex Size: 1Defense: 10Dmg: 11-13Spd: 6 (swift)
Special: death cloud range attack - damages living creatures on adjacent hexes to target.
Cloud won't damage your undead units, so aim right in that crowd!
Power Lich
Power LichHealth: 40Attack: 13Shots: 24Movement: ground
600 gold (3)Hex Size: 1Defense: 10Dmg: 11-15Spd: 7 (extra swift)
Special: death cloud range attack - damages living creatures on adjacent hexes to target.
Now they last longer and are able to do more damage! A must for good necropolis army.
Black Knight
Black KnightHealth: 120Attack: 16Shots: noneMovement: ground
1200 gold (2)Hex Size: 2Defense: 16Dmg: 15-30Spd: 7 (extra swift)
Special: 20% chance to curse enemy.
Awesome ground unit. As any undead it cannot be blinded, so your enemies will have to look out.
Dread Knight
Dread KnightHealth: 120Attack: 18Shots: noneMovement: ground
1500 gold (2)Hex Size: 2Defense: 18Dmg: 15-30Spd: 9 (ultra swift)
Special: 20% chance to curse enemy ; 20% chance to do double damage.
I think it's the best lvl6 unit in the game! Double damage ability puts Dread Knights above Naga Queens.
Bone Dragon
Bone DragonHealth: 150Attack: 17Shots: noneMovement: flying
1800 gold (1)Hex Size: 2Defense: 15Dmg: 25-50Spd: 9 (ultra swift)
Special: -1 to enemy morale.
They are truly fearsome for enemies with low morale. Simply keeping them on battlefield scares enemies.
Ghost Dragon
Ghost DragonHealth: 200Attack: 19Shots: noneMovement: flying
3000g + 1m (1)Hex Size: 2Defense: 17Dmg: 25-50Spd: 14 (ultra quick)
Special: -1 to enemy morale ; 20% chance to age enemy (halve hit points of all stack members).
When situation seems hopeless, take a chance on the best enemy stack! If you'll get lucky, half their hit points will be gone instantly!! Ageing ability makes ghost dragons as dangerous as other lvl7 creatures.