UNDEAD
There are three heroes for each race, heroes gain experience
for each creature that dies on the battlefield. Heroes gain
levels with this
experience and also acquire skills. Here are the undead heroes:
Lich
Death Knight
Dread Lord

Undead Strategy
An easy early strategy for using undead is using crypt fiends. Crypt
fiends are easily created and can be created early in the game. It's only
requirements are the Crypt (where it is trained) and the Graveyard (where
it's upgrades can be researched). I suggest making two or three Crypts to
speed up your units production. It is essential to level up your hero in
the early stages of the game and Crypt Fiends are the perfect units to
accompany it. In late game if your Crypt Fiends cannot get through your
opponents defenses, try using Frost Wyrms to rip apart his or her mineral
gathering units. Fly right through or anound their defenses and park the
wyrms right above the gold mine. This will REALLY tick off your opponent
so be ready for a counter-attack. After this your opponent will be
preoccupied setting up defenses around his mine. Use this to your
advantage and bring in some meat wagons and blow apart his or her towers
or whatever it is that is stopping your fiends (make sure you have
something protecting your wagons they are quite weak). After his defenses
are
crippled bring in the fiends and maybe some Abominations to destroy his
units. Follow this with a second wave of Meat Wagons to destroy the rest
of his buildings.
Each hero is equal in strength but I prefer the Death Knight. He can
heal your units and damage the enemy with Death Coil, and his Unholy Aura
will help your Fiends recharge their life faster. He can also heal
himself with his Death Pact (you must sacrifice a unit to use this), and
his ultimate skill, Animate Dead, can turn the tide in your favor in a
tough battle.