Sludge Vohaul is a relative of Slouch Vohaul and Slash Vohaul (unofficial). He is a cyberneticist and robotics engineer. His minions include Sariens, Apemen Guerillas, Sequel Police and possibly Aeliens.
Background
He is known for essentially stealing the work of both his 'brothers?'. This included sending the Guerillas to capture the clone army (now Insurance Salesman (unofficial)) from the remnants of the Aelien forces and invading Labion from his secret Asteroid Base (unofficial) and sending the Sariens to steal the Star Generator project. Both of his brothers were apparently killed during the attacks. Although the final end of Slouch is a bit of a mystery (R.A.L.P.H. may have gotten to him, or Sludge might have 'gotten' to him). Both of Sludge's plans were thwarted by Roger Wilco (unofficial).
Sludge would later attempt to resurrect the Insurance Salesman clones technology to take revenge on Xenon (unofficial). But was thwarted once again by Roger Wilco. His life support was cut, and he died. But he was able to backup his mind to a computer disk, and send it off his Asteroid Base.
Years later, the disk made it into the hands of the Xenon scientist working on the Xenon Super Computer. His mind was unleashed as a virus, taking over the system. Creating Sequel Police (unofficial), he sent them through various time eras, searching for Roger Wilco, to take revenge on him. This turned out to be a mistake, as Roger was brought to the future by his own son, and found a way to format the computer, and destroy Sludge's plans again.
However, unbeknownst to Roger Wilco, two Sequel Police were able to create a backup of Sludge Vohaul in time, and send it back into the past, shortly before the super computer went into meltdown. The new disk was sent back in time to his Apeman (unofficial) minions, who built a new body for the villain. He prepared his newest plan to take revenge on the hapless janitor in the past.
VSB: Paragon
VSB: Renegade
Roger Wilco leaves Sludge Vohaul to die. His robotic body is shot by the Apemen across the galaxy to a water planet. He was rebuilt by the Space Monkeys (unofficial) living there, to become their leader. He discovers he cannot escape as they have no ships. Sludge Vohaul vowed to take revenge on Roger Wilco.
Sometime in the future, Chronarr escapes Xenon with a copy of Vohaul's mind just as the planet is destroyed by a supernova.
Incinerations
Dr. Sludge Vohaul was once one of Xenon's top scientists, before turning to a life of mad science. His claim to fame was developing the Star Generator (unofficial) that was used to reignite Xenon's sun. He was also the creator of the Digital Personality Conversion, which he used to make back-ups of his own mind which were sent across the hypernet upon his immediate death in an asteroid explosion. Months later, systems across the galaxy were infected by the Vohaul Virus (unofficial) and he began commandeering military networks, enslaving races, and continuing his post-mortem plands of conquest. Turgis and Bruce Havoc spent seven years hunting down formating every known copy of him.
An evil time assassin Chronarr found the last copy of the Vohaul Virus, apparently in the Space Quest XII (unofficial) era. Stealing it, he left shortly before Xenon was destroyed by a vast detonation. Chronarr made copies of Vohaul's mind converting it into the BENI program, and distributing around the galaxy. He also took from Vohaul the plans for the Star Generator 2.0, the Double-Zero virus, and the Delta Storm Tracker. He then created a new False Vohaul to run the every day functions of his base. A copy of the original vohaul remained trapped inside of a computer inside of a closet. His mind had become fragmented over time, though still one being, each part began to speak in unison.
Sludge Vohaul asked Roger to help him defeat Chronarr as he was a common enemy shared by both.
Behind the scenes
It is possible that Slouch was a false idenity used by Sludge in order to get Xenon's permission to continue his research. In which case, Slouch didn't actually die, he never existed to begin with... This would mean that the Aeliens were always under orders by Sludge himself! When the Aeliens failed their mission, he sent in the Guerillas instead.
Vohaul Strikes Back loosely acknowledges the events of fan games SQ0, SQ:TLC, and SQ4.5 (as well as the six official games), and possibly the unofficial SQ2 remake, as having taken place before events of the game. The above synopsis takes this into account. Although there may be 'discrepencies' between the games.
Space Quest: Incinerations appears to take an alternate history route of Sludge's past, ignoring Slash Vohaul completely. In this timeline sludge was the developer of the Star Generator, and upon his defeat at the end of events of SQ2, he spread his mind to more than just one disk.
Presumably, in the Renegade/Incinerations timeline, the Space Monkey Vohaul was found and destroyed by Aries Command or Bruce Havoc. It is less likely that Roger Wilco encountered him, as he mentions only having defeated Vohaul a few times before events of Incinerations. It is mentioned that all copies of Vohaul were likely destroyed by Aries.