Dereth was a member of the Vidiian Sodality, a culture who were wracked with a disease known as the phage. He took the position of "honatta" to another Vidiian, Motura. His task was to locate organs to replace Motura's own diseased organs. While the pair took from the dead where possible, Dereth was quite prepared to take organs from live subjects if necessary.
The pair had set up camp on an asteroid, which was discovered by Voyager's crew. Dereth attacked Neelix and used a device to transport his lungs out of his body, leaving him for dead. When Voyager personnel discovered a way to penetrate the forcefield shielding their hideout, Dereth and Motura fled in their own ship and took refuge in an asteroid field. However, Voyager was able to locate them and beam the pair aboard.
Dereth and Motura explained their races' situation to Janeway, including the fact that Dereth had already transplanted the lungs into Motura. Janeway was unwilling to kill him to save Neelix, so decided to let the pair go with a warning that if the Vidiians came after her people again they would be met with lethal force. Dereth was happy to leave it at that, but Motura asked him to help. Since Vidiian medical techniques were more advanced than the Federation's, Dereth was able to transplant one of Kes' lungs into Neelix, saving him.