Unless otherwise stated all effects are passive.

Effects stated are positive such as -10% Melee Damage which means that unit reduces (melee) damage taken by the stated amount.

Undead has a few unique abilities that the game fails to document properly!

Hatred is a measure of how likely the enemy is to attack you which is pointless as the AI always aims for your archers first and human players can attack whomever they want.

Blind is a stun and the most OP skill in the game!

Sentinel
-10% Melee Damage, -10% Ranged Damage

Crossbowman
-10 Armor, -20 Armor (if enemy is within 6 tiles)

Swordsman
Attack Twice or Counter-Attack Twice (whichever comes first), +10 Attack (First 5 rounds)

Griffin
Unlimited Counter Attacks, 5% Counter Damage

Acolyte
No Close Range (Melee) Penalty, Remove Buffs from Target

Knight
+Melee Damage for every tile moved prior to attack, +2 (Melee) Attack Range

Angel
Heal if enemy uses "wait", Inflict random status effect on attack

Skeleton Soldier
-1 Enemy Move, -10% Ranged Damage

Zombie
Lower Attack & Defense, "Paralyze Poison" (No effect listed!)

Spectre
"Self Regeneraion" (No effect listed!), -10% Melee Damage

Vampire
Damage Dealt Restore Life, Avoid Damage Once

Lich
AoE Attack (2x2), Armor Penetration based on quantity of Undead units present

Abomination
"Mad Massacre" (No effect listed!), Curse enemy when their troop count lowers.

Bone Dragon
Reduce Enemy Life an extra 20%, Melee Range +2

Demonic Child
Lower random Enemy Units Defense and Increase Own Defense when moving, +5 Damage (First 3 rounds)

Succubus
Counter any attack with a Ranged Attack, Target locked when attacked

Hell Hound
Melee attack is AoE, Enemy cannot counter-attack

Goblin of Hell
Defense down and Hatred up on random enemy units, Attack up when attacked

Horse of Hell
Cast fear(no effect listed!) when enemy units move, Attack & Defense up when hero casts negative status effect spells

Beast of the Abyss
Increase Morale on allies turn only (only works on Hell troops), Mark an enemy when they move (no effect listed!)

Devil
Attack increases when it attacks, Attack increases when it is attacked

Essense of Nature
1st turn +4 Move Deal -40% Damage, Speed -6

Dwarf
-10% Magic Damage, +50% avoid enemy hero magic

Elven Archer
Shoots twice, Lowers chance of enemy choosing to attack them (has no effect on manual targetting/players)

Centaur
+15% Damage to ranged units, attack at range before moving

Dryad
Prevent enemy moving on attack, +50% Defense and unlimited counter when defense is used

Unicorn
Blind enemy on attack (stun), Attack & Defense increased when hero casts positive buffs/spells

Stone Golem
Attack & Defense increased on moving/attacking, allies Defense increased when moving/attacking

Hell #1
Regarded as the strongest faction in the game due to both Succubus' ability to counter attack enemy ranged attackers and furthermore the Hell Hounds ability to attack without being countered.
A morale increase is an amazing ability and pushes Hell into the realm of broken.
Hell units inflicting debuffs by virtue of nothing more than being active is also empowering.
Overall, Hell units have a very easy time scoring kills.

Forest #2
Almostly completely ignored by many players but almost tied with Hell in terms of strength.
The ability to shoot twice at range is good but sadly Elven Archers have low base attack. Unicorns ability to stun a unit and keep them out of combat is highly valuble and combined with Trents ability to prevent a foe from moving, offers a great way to lock down 2 threatening enemy units. Sprites ability on turn 1 to cross the battlefield is amazing but they deal 40% less damage(on that turn) and they also have low base attack damage too so more often than not they take huge losses instead.
Focus more on locking enemy units down rather than outright brute force.

Fortress #3
Fortress are just outclasses by Hell and Forest. They offer nothing of significant value except their base units hit very hard and that is about it. They can take a beating but sadly the enemy can dish it out faster than they can endure it. Priest ability to remove buffs is nice but Hell functions on debuffs so its worthless and priests are already fragile which further does little to help.

Undead #4
Too many abilities are unlisted to know what they even do, never mind if they are good or bad, debuffs or spells, its just a guessing game.
That aside, the skills we do know about are really lackluster. Undead brings nothing to the table that the other factions can't do (but better).