Orcus is a mage who got sent to the underworld when a banish spell went wrong. He did not die, he just banished HIMSELF to the Underworld instead of the demon that he wanted to send to the Underworld. He set about making a place for himself there, because he wasn't aware that one could get back. He got entrenched and made a name for himself. By the time that he found out that return was possible he had lots of minions and had started on several large research tasks knowing that he now had eternity to finish the research instead of just some 100 mortal years. However as part of this research he requires lots of different things from all over the plane, instead of sending retainers he sends adventurers and rewards them with knowledge about how the plane works, the spirit skills, portals etc. Like King Arthur he wanders around between castles with his retainers and 'knights', so you don't always know exactly where he is. Spirits in the underworld are physical, so spirit stuff is 'physical' to other spirit stuff.
Back on the Prime, most people don't know about him, or don't believe in him. The people who have been "resurrected" propably met him, but most of their friends will probably just regard it as barmy screed. A load of tief from some berk who was the subject of a miracle. Necromancers will likely believe in him, however, even though others may think of him as a faerie tale, they tend to give a lot of credit to people who experienced death itself and came back to tell the tale. Oink. He's able to get back to the Prime like any other spirit, and may even be able to open a portal that he can exit through. However, he doesn't want to. He has everything he needs to the Underworld, including immortality, and going back to the Prime wouldn't be worth it at all. He has developed a spell that allows him to contact people on the Prime in their sleep as well. It is said that he has always been facinated with creating life. It is also said by some that he is slightly insane and has a strange facination with Death, but that is, of course, unconfirmed.