The Fourth Overlord, also known as The Overlad or Witch-boy by Kelda (although it was originally a term used to mock him as a child) and the Demon Lord of Nordberg, was, before his presumed death, the ruler of the world.
History
Early life
After his father, the Third Overlord, was trapped in the Infernal Abyss, his mother left the Dark Tower and abandoned him outside the gates of Nordberg. The empire of the Third Overlord started to fall into chaos without its iron fist leadership, until its complete annilhation due to the Great Cataclysm that destroyed the lands around the Dark Tower, leaving behind the Wasteland.
Overlord II
Several years later, still a young child, he was found by a group of brown Minions on the outskirts of the town. The people there greatly disliked and feared him, likely due to his magical abilities, except for Kelda, who already had a mild crush on him, despite his status as an outsider. During the Midwinter's Eve festival, the Glorious Empire sent a force of soldiers to Nordberg, threatening to destroy the town if they didn't surrender all their magic users. The villagers threw the Witch-boy over the walls, but he was able to escape capture thanks to the Minions. He was then raised by Minion Master Gnarl. As he was raised in Netherworld Tower, the Glorious Empire rose to power.
When he came of age, after thirteen years in the Netherworld with the Minions, he assumed his rightful place as Overlord. Shortly after proving he was Overlord, he conquered Nordberg in revenge of the villagers casting him out to the empire. There he defeated the local governor,Borius and took his first mistress, Kelda.
He then proceeded to conquer the Empire held Everlight, where he defeated the town's governess, another agent working for the Empire, and rescued Juno, another mistress, from Everlight.
Next, he traveled to the Wastelands, a barren land destroyed by the Great Cataclysm, and found its sanctuary. Here, he joined the elves and drained Fay, their queen, to power the Tower Heart. The Overlord then took Fay, dead or alive, as his third and final mistress.
He finally launched a major strike against the Empire, smashing through its anti-magic shield by catapulting the Tower Heart at the walls of the Imperial City. He proceeded to conquer the Empire Heartlands. A mysterious prophet (later revealed as the once-mistress Rose) appeared and predicted he would share the fate as his father (the Third Overlord) if he followed on the same path, bringing him to some sort of exile or gory death. She eventually allies herself with him as he proceeds to conquer Empire City, stating that Solarius has become corrupt and that the Overlord's Darkness is necessary.
After finding Solarius and discovering that he is actually Florian Greenheart (a servant of queen Fay and the cause of the Great Cataclysm), Rose informs him that he has gone mad with power and must be stopped before he can destroy all traces of good in him. Solarius then jumped into a great vat where he had been storing all the magical energies his Empire had been gathering from accross the world and transformed into the Devourer. After fighting the newly formed Devourer, the Overlord realizes that in order for there to be evil, there must be good, and the only thing that could destroy a great evil is an even greater evil. The Overlord accepts that he is that greater evil. After killing the Devourer, the Overlord destroys the "Glorious Empire" and takes his rightful place as ruler of all the land as Overlord.
But as Gnarl says, his father, the Third Overlord, now a god, will find a way to escape the Infernal Abyss somehow.
Overlord: Fellowship of Evil
The Overlord was eventually destroyed by the Shining Justice, along with his empire. One of the four Netherghuls:Inferna, Cryos, Hakon or Malady are each fighting to take their place as Overlord.
Netherworld Tower Upgrades
Even though the Overlord may pay for an abundance of decorative upgrades for the tower, concerning his mistresses , he only has one such addition.
Tower Upgrade 
Overlord Banner 100
Personality and Traits
He seems to be interested in women more than his father; this may be due to his young age, or could be because his father liked them as well, but because the dark tower could not host 2 mistresses at once, he had to choose only one.
Unlike his father, his skin appears to be blue and covered in spiraling symbols. The reason is perhaps, he was contaminated from the Great Cataclysm when the Tower Heart exploded. Nordberg is apparently the first safe area after the Wasteland, and the villagers don't appear to have suffered the plague. Kelda says that she heard that 'the cataclysm was horrible, thousands of corpses had to be thrown away' indicating that Nordberg was never hit by the plague. Another theory is that the overlord was contaminated shortly after being born, and was probably still was in the tower lands shortly after the explosion), considering the plague victims have similar skin color and markings as he does, However, unlike the plague victims he developed a propensity with magic and he could control it freely. It could be that his parents could use magic, and his blue skin is a side effect.
Overlad in his childhood
Even as a child, the Fourth Overlord was incredibly strong; able to beat a legion soldier to death in two blows using a club. When he grew older, he took on a much larger and more muscular form than his father, and was capable of landing several punishing blows in quick succession, each one powerful enough to shatter Empire legionnaire armor.
In terms of overall evilness, the Fourth Overlord was much more efficient and domineering than his father. Whereas the Third Overlord relied on his own charisma.deeds, and seemingly miraculous timing to keep his subjects in line, the Fourth Overlord often chose to simply drain their essence until they were little more than puppets (or would simply kill them off without hesitation).
He also took a more "hands on" approach to his conquered lands, keeping a cadre of minions topside at all times whereas the Third Overlord only did so in specific cases. However, he was not outright cruel, and many commoners from many lands he owned felt no fear in alerting him to the happenings of his domain (and asking for very stupid things).
His style of waging war greatly contrasted the styles of the previous overlords before him. Again, the most notable contrast is with his father, the Third Overlord. Whereas the Third Overlord preferred battles of attrition and full frontal assaults to overwhelm and capture enemy points (such as his assault on the Silent Order), the Fourth Overlord was a master of subterfuge and blitzkrieg. For example, in the Nordberg commune, instead of facing its dryad guardians head-on, the Fourth Overlord (in the body of a minion) snuck around via a smaller branch and attacked the surprised dryad from the rear. When conquering Everlight, the Overlord quickly swept through the town, scattering and confusing the static legion phalanges with attacks from multiple directions and speedy pushes, ultimately defeating the much more numerous and arguably more experienced Empire forces.
He had the natural ability to control magic, though like his father still needed spell stones to activate his full magical potential. He is also, unlike the Third Overlord, "in tune" with arcane energy--to the point where even standing near a netherworld gate regenerates any wounds he suffered and replenishes his internal mana reserve. This is possibly due to the fact that he is the son of the Third Overlord, who possessed magical ability of some description, or it could be the Tower Heart's explosion infused him with some of the original spell stone power (since the Tower Heart managed the spells). He seemed to be more attached to the Minions then his father, considering them to be close friends, since they were his only ones who didn't fear him, treating him like one of them in, contrasting against the Nordberg children who despised and feared him. He grew up with the Minions, which is the possible reason he was willing to sacrifice the newborns in order to bring some of his more experienced (and likely more friendly towards) dead Minions back to life through Mortis, an ancient blue Minion responsible with the life and death of the Minions.
References and Notes
Gallery
