A Kandorian crime lord who battles the team of Nightwing and Flamebird and is in actuality a counterpart of Jor-El whose "Krypton" is believed to never have exploded. Jur-Ll was "his Krypton's" greatest scientist, and was considered by the authorities to be its greatest threat. In his hands, the simplest of materials became a master weapon, which he used to build a few simple men into criminal empire to rule "Krypton." He would indeed have ruled his planet had his chief lieutenant not betrayed him and turned him over to the authorities. As punishment for his crimes, he is condemned to permanent exile within his own version of the "Phantom Zone."
Jur-Ll would have remained there indefinitely had he not escaped through a dimensional rift that led him to the Kandor of the Earth-1 universe. However, he seemingly trades one prison for another, as he arrives in Kandor on the very day that Brainiac steals the city for his collection (SF No. 189, May/Jun 1978: The Crime-Lord of Kandor!").
Jur-Ll does not let this deter him and he sets about building a new criminal empire with the ultimate goal of escaping Kandor (revealed in SF No. 193, Jan/Feb 1979: Target: Van-Zee!"). Years later, he battles the team of Nightwing and Flamebird, but is defeated, and apparently killed, when Flamebird tosses him into the devices that he is attempting to use to destroy the bottled city and re-enlarge himself (SF No. 194/2, Mar/Apr 1979: "Showdown!").
(Note: It is possible that Jur-Ll's home universe is that of Earth-3, where roles are often reversed [for example, President John Wilkes Booth was assassinated by actor Abraham Lincoln, and that he is the father of the Earth-3 criminal Ultraman. However, Jur-Ll dies before being apprehended by Nightwing and Flamebird and the chronicles never specify this relationship.)