The Overlord was resurrected by Gnarl and a group of minions after the pervious Overlord was removed due to the seven heroes. After being resurrected the Overlord was given control of a small group of minions and tasked with restoring his tower and showing everyone that the Overlord has returned and that the new Overlord was going to take out the corrupt heroes of the land.
At the end of the game, The Wizard (the Original Overlord) reveals to The Overlord that he was once a Hero, who joined up with the other Heroes to defeat him. He fell from the Tower however, and his Hero companions assumed he was dead. The Wizard had Gnarl and his Minions resurrect The Overlord to defeat his Hero Companions, that have became corrupted.
In Overlord: Raising Hell he leads a campaign into the Abyss where his servants have ran away to. At the end he manages to slay The Forgotten God but is ultimately trapped in the Abyss, though now he rules as it's new lord. Up in the real world his Mistress (revealed to be Rose in the sequel) is pregnant with his son, the protagonist of Overlord II.
The Overlord is able to use several weapons, but only one can be equipped at a time. Once the ability to forge weapons and armor has been restored to the tower the Overlord is allowed to upgrade weapons and his armor. To due this it cost gold and sacrificing minions as well. The Overlord has the choice between three weapons: a sword, a mace, and an axe. Each can be forged with different metals same goes for the armor. The different metals allow stronger and better weapons.
The Overlord has complete control over his minions. The minions are happy to kill and destroy for the Overlord. Minions will get his treasure and even equip themselves with weapons.