Kol Thezar was the Cheif Dreadlord under The Black demon.
The First Invasion
During the first invasion Kol Thezar operated in secret, (using temporarey corpareal bodies made by the Lich Rage Frostchill) sabotaging the Human defenses and aiding the orcs.
The Second Invasion
With Rage's death at the battle of Mt. Snowmourn, Kol could not stay in the mortal world. Instead he possessed various Orcs close to Thrahk to protect him. Upon arrival through a Demon Gate, Kol killed the warcheif and the majority of the Orc army.
The seige of the Twisted Citadel
10 years passed and Kol warped Belemonde beyond recignition. The humans however struck back, ammassing a huge army of humans, elves and dwarves to take back their land.
Human infiltrators destroyed the demon gate however and Kol was left only with his personal bodyguard. Needless to say the alliance took back the land and killed Kol Thezar.