He is an evil sorcerer who feeds off the life force of other people's pain and suffering. He has a non-corporeal form and is 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 forces 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 is able to cause so much pain to Maldis that he takes corporeal form. After Crichton hits him in the face, he disappears in a red light.
The crew encounters him again in "Picture If You Will." He takes the form of Kyvan, who sells a painting to Chiana. The painting is a portal between the realm were Maldis exists and the realm with Moya. He plans to use the painting to get his revenge on those that defeated him. However, Zhaan manages to use her powers against him and defeat him once again, although she is left wondering if he might return.