Discworld MUD

Discworld MUD opened in 1991 on the LPMud family, which means the world is written in LPC and can be extended by its own player "creators". Ankh-Morpork, Lancre and the rest of the map were rebuilt street by street, with Pratchett's permission and with the bureaucratic absurdity the source material demands. The game is known for an unusually detailed skill and language system, and for the fact that Death does, of course, come round and talk to you in capitals.

Its mudlib was released and became the basis for other LPMuds. The game is still running, which makes it older than most things that have ever been called an MMO.