The Undead

Warlords: Call to Arms
They are described in Warlords: Call to Arms as "Once resurrected by a great necromancer who has since perished, these warrior will not die through natural causes. Only through the weapons of war."

Their special unit is the Flailer.

They border Orcs of the South, Mountain Trolls, The Demons, Human Alliance & Men of the West.

Warlords 2: Rise of Demons
They are described in Warlords 2: Rise of Demons as "Diseased scythe style halberds are very effective against melee units."

Warlords: Epic Conflict
They are described in Warlords: Epic Conquest as "A race created by a long dead necromancer. Their strength is in their numbers."

Lore
Note, the following is fanon (fan-canon) and is NOT to be considered absolute canon