Dorian 5 was a planet which used to be the home of an unknown alien species. This species was exterminated, along with the planet, by The Vindicators in order to kill Doom-Nomitron.
Prior to the events in episode Vindicators 3: The Return of Worldender, The Vindicators had assembled without Rick Sanchez and Morty Smith, due to conflicting personalities, to defeat Doom-Nomitron. Calypso, Diablo Verde and Lady Katana died fighting Doom-Nomitron, possibly on Dorian 5. Doom-Nomitron, using his shape-shifting powers, hid out among the Dorian 5-population. Not being able to distinguish him from the native aliens, The Vindicators took to exterminating the whole planet to make sure they killed Doom-Nomitron.
Dorian 5 is first mentioned by Crocubot in the second Saw-like trial by Rick on Worldender's ship. Alan Railselaborates that they exterminated the entire planet, while Supernova justifies that they had to kill Doom-Nomitron. Rick casually mentions he could have made a device to detect Doom-Nomitron from orbit in a second.