The Deindum were a species who, if allowed time travel, would have rivalled the Time Lords.
Biology
The Deindum were originally reptilian but evolved to be made of blue/green light. (AUDIO: Resurrecting the Past, Escaping the Future)
Culture
The Deindum honoured no pacts, because they trusted nobody but themselves. (PROSE: Present Danger) They had no interest in writing, but were interested in pictures. (PROSE: Winging It)
Technology
The Deindum used masers as weapons. (PROSE: Big Bang Generation) They used weapons that shot flame on Maximediras. (PROSE: Present Danger)
Some Deindum had skull turrets grafted onto their skulls, which resembled a third eye. (PROSE: The War of Art)
The Deindum were skilled at constructing androids, disguising them heavily with organic parts. (AUDIO: Resurrecting the Past, PROSE: Present Danger)
History
In the 27th century, the Deindum declared war on the universe, beginning what became known as the Deindum War. (PROSE: Adorable Illusion) They first showed themselves to the universe on Maximediras. (PROSE: Present Danger)
Irving Braxiatel and Doggles attempted on several occasions to destroy the Deindum before they evolved into sentient creatures, but on each occasion they were stopped by Deindum from the future. (AUDIO: Escaping the Future)
Braxiatel destroyed a Deindum armada in the Battle of Capra. (PROSE: Winging It)
The Deindum tried to make Deimos and Phobos leave Mars' orbit in the 21st century. Merlin sent Excalibur to Winifred Bambera and Ancelyn in order for them to use the sword to prevent the Deindum's plan from succeeding. (PROSE: Excalibur of Mars)
Bernice Summerfield discovered that during the industrial age of the Deindum's development, they were contacted by Deindum from the future and were given time travel technology. Shortly after they gained the ability to travel in time, the Deindum's biology was altered by Braxiatel to make them a weaker species.
With time travel, the Deindum conquered the universe and wiped out the human race.
Because their appearance was different when the Deindum contacted their past, Bernice Summerfield and Braxiatel manipulated the situation to portray the future Deindum as con artists and a war broke out between the future and past Diendum. The war caused the Deindum to be destroyed at a point before they developed time travel and thus failed to conquer the universe. (AUDIO: Escaping the Future)
Rather than wiping the Deindum out entirely, Bernice Summerfield's actions reduced the Deindum from a set of ultra-savage warmongers to a small set of skirmishers. The war was considered to pretty much won after the the Battle of the Pakhar Shipyards and later the Defence of the Tsarissian Drydocks. (PROSE: Adorable Illusion)