The first Extremists originated on the other-dimensional world of Angor.
The first Extremists were killed in a nuclear event that they started themselves, along with the rest of their world. However, robotic duplicates of the Extremists were created as part of a theme park attraction. The robots, modeled too closely after the violent originals, eventually went berserk. They tracked the surviving
Champions of Angor to Earth, and tried to destroy the planet in the same way they destroyed Angor. They were defeated by the Champions and the heroes of the Justice League Europe . of the Extremists,
Dreamslayer, was later shown not to be a robot, having survived the destruction of his world due to his mystical nature. Instead of being deactivated, he was instead defeated by the Silver Sorceress.
The Extremist robots were taken to "Madame Clouseau's" in London. They were later reactivated by Dreamslayer, who was possessing Maxwell Lord at the time. Dreamslayer was again defeated by the Sorceress, though at the cost of her own life. The defeat of Dreamslayer deactivated the other Extremists.
Later, Queen Beatriz of Bialya tries to create an army based on the Extremists' technology, but her plans are foiled by Captain Atom and Extreme Justice
While he was trapped in the Astral Plane, Dreamslayer's services were acquired by the Overmaster. Together, they assembled a new group called the New Extremists. After this, Overmaster imprisoned Dreamslayer in the Astral plane again. The New Extremists became part of Overmaster's Cadre.
Original Extremists Return
The Extremist robots would later be rebuilt and reactivated several times, fighting notable foes such as Supergirl, Power Girl and the JLA.
On Earth-8, the Extremists are a group of villainous outlaws who have refused to comply with a law stating that all metahumans need to register with the government (analogous to the way certain Marvel heroes behaved during the Civil War event).
They battle against the Meta Militia, a group led by Tin Man (a parody of Iron Man), and currently reside in a captured country called Slovekia.
They were encountered by Donna Troy, Jason Todd, " Bob" the Monitor, and Kyle Rayner while the heroes were searching for Ray Palmer.
Lord Havok would later refuse to join Monarch's Army , leading to an encounter where he and Monarch fought, allowing Lord Havok to steal some of Monarch's power for later use.
The original Extremist (less Dr Diehard) made an appearance in the chapter "Shadow of Hawkman" from the Justice League Unlimited Animated series. In the prologue of the story, they were battling Hawkgirl, Green Lantern, Batman and Vixen.