The Dark Circle is a fictional criminal organization in that appears in comic books from DC Comics, primarily part of the Legion of Super-Heroes comic books. They first appeared in 1968, created by Jim Shooter as a criminal organization founded by five members and populated by a multitude of clones of the original five members.
The Dark Circle concept was later modified to consist of members from five core worlds instead of clones. A later version, after DC Comics rebooted their history, was led by Brainiac 4 and had several known Legion of Super-Heroes enemies among their ranks instead of the previous generic masked henchmen.
The Dark Circle first appeared in Adventure Comics #367 as an insurgent group planning to conquer the United Planets in the 30th century. It was composed of only five people and armies of clones created from those five people. The Dark Circle remained mostly shrouded in mystery, even with its defeat by the Legion of Super-Heroes.
They launched a direct invasion of Earth during the Legion's Adventure Comics run, following an attack by the Fatal Five that had left the Legion and Earth's defenses devastated, and were in the process of overwhelming both when Brainiac 5 discovered the Miracle Machine and used it to repulse the attack, returning all the Dark Circle soldiers to their homeworlds.
They then sought to invade Earth indirectly through others, most notably the Khunds and Dominators whom they manipulated into what became known as the Earthwar. As part of their machinations, they also released Mordru, hoping to use him against the Legion. This proved to be a mistake, however, as Mordru instead gained control of the Dark Circle. This was not immediately apparent, though, as he continued with the plans they had already drawn up. Eventually Mordru and the Circle were defeated.