War Lord

The War Lords were a humanoid race who tried to conquer Mutter's Spiral at least twice.

Biology
War Lords looked identical to humans and could easily pass themselves off as humans which allowed them to assume positions of command to fulfil their agenda. (PROSE: Timewyrm: Exodus) However, they were highly sensitive to intense light in comparison, reacting to it with extreme pain. (TV: The War Games)

Technology
The War Lords used special eyeglasses disguised as ordinary spectacles — or, in the case of von Weich, a monocle — to induce mind control in the field, which they used these to influence the behaviour of human soldiers. They would put these on and speak in order to affect the minds of others. With these glasses, they could hypnotise humans into forgetting the materialisation of a SIDRAT. On their base, they had technology to re-condition and re-process the minds of human soldiers and machines to tell whether or not the processing had taken. (TV: The War Games)

In their efforts to assist Nazi Germany win World War II, the War Lords used their technology to control SS soldiers, to the point where they were able to reanimate their corpses. They set up a transmat booth to transport people from Berlin to Drachensberg, and a nuclear reactor underneath the castle. They also had a telepathic receiver shaped like a crystal ball which was used to contact the Seventh Doctor, and a powerful laser weapon disguised as a cane. The War Chief also gave them time scanning technology and SIDRATs. (PROSE: Timewyrm: Exodus)

History
When the War Chief first gave them time technology, the War Lords abducted members of several different warlike races and experimented on them to determine their suitability as soldiers in their armies; one such individual was Ossu-male, an Ulk-Ra. Eventually, the War Lords decided to use humans for their army, eventually coming to see that humans were more dangerous. (PROSE: War Crimes)

The War Lords attempted to build an army by taking human soldiers from different moments in Earth's history and forcing them to fight each other; the survivors would be used to conquer the galaxy. This plan was defeated by the Second Doctor and his allies, and by the Time Lords, who executed their leader and placed their home planet within a force field — cutting it off from the rest of the Universe forever. (TV: The War Games)

The War Lords tried to kill the War Chief, who barely survived after a failed regeneration and was taken back to their homeworld, where he was kept as a scientific oddity. He eventually convinced the War Lords that he hadn't betrayed them, and began work on a new plan for galactic conquest. The War Lords escaped from their time-looped planet and travelled to Nazi Germany. They set themselves up as the Black Coven and allied with Adolf Hitler (who possessed the power of the Timewyrm), supplying advanced technology disguised as black magic to help win World War II.

This plan was uncovered by the Seventh Doctor, who had Hermann Goering and his men attack their base in Drachensberg, killing them. The Doctor then overloaded their nuclear reactor, causing it to explode and destroying their advanced technology. (PROSE: Timewyrm: Exodus)

Alternate timeline
In an alternate timeline created when the Master used a conceptual bomb to destroy the Doctor's TARDIS, so that the Doctor never left Gallifrey, the War Lords' armies spread across the galaxy. (AUDIO: The Light at the End)

Behind the scenes

 * The species is only referred to as "the Aliens" in The War Games, where "the War Lord" is only ever used as the title for the species' leader. This was echoed in Destiny of the Doctors and Divided Loyalties.
 * Gareth Kavanagh stated in 2021 that the original plan for Omega was to have Omega meet the War Lords, but that the licensing deal couldn't be worked out. In the end, the finished product instead featured the Minyans instead.