He is a evil sorcerer that feeds of the life force of other peoples pain and suffering. He have reach a non-corporeal form and are able to take any form he desires, but it also makes it more or less impossible to kill him.
Moya's crew first encounters him on a merchant planet were he traps John Crichton's soul and foreces him to face Crais. His plan is to have Crais hate Crichton even more and be more destructive in his search for Crichton, and that way he would be able to feed on the suffering Crais creates. However Zhaan manages to re-discover a side of herself that she fears and are able to cause so much pain to Maldis that he takes corporeal form and Crichton are able to hit him, after a hit to the face Maldis disappear in a red light.
The crew encounters him again in the form of Kyvan, that sells a painting to Chiana, the painting is a portal between the realm were Maldis exit and the realm with Moya. He plans to use the painting to get his revenge on those that defeated him. However Zhaan manges to use her powers against him again and defeat him once again, however she is unsure if he is able to return again.