Asmodeus was the name of the demonic alien creature that possessed Babylon 5 security officer Simon Burke while on a vacation to Earth in 2271. He was portrayed by Bruce Ramsay in Babylon 5: The Lost Tales: Over Here segment.
When Father Cassidy and Colonel Lochley interviewed Burke in his holding cell on Babylon 5, he revealed that Burke's consciousness had been suppressed and the entity which had possessed Burke was named Asmodeus. Asmodeus claims his race to have been placed in deep space by God as a sign of the existence of demons and thus of other supernatural phenomena, to restore faith in a universe that had grown proud of its scientific achievements. In fact Asmodeus and the rest of his kind had been banished to Earth, and the only way he would be able to travel the stars again in his native form was to possess a body on Earth, take a transport to deep space (in this case, Babylon 5), and be exorcised there. After Lochley and Cassidy realise this they confront Burke and then decide to send Burke back to Earth where Asmodeus will be exorcised and hopefully Burke can resume his normal life.
Asmodeus may have died together with his kind in the Earth year 1002262 if he hasn't escaped before that happened.