Monster (VMS)
A 1988 multi-user adventure in which players built the world themselves — a year before TinyMUD made the same idea famous.
Rich Skrenta wrote Monster in Pascal on a VAX at Northwestern University. It was a text adventure for several simultaneous users on the same VMS system, but with one decisive difference from most MUDs in the Essex tradition: anyone could create rooms, objects and descriptions from inside the game. The world wasn't a finished course to get through but a place that grew while you were in it.
Being tied to VMS limited its reach, so TinyMUD got the credit for user-built worlds when it arrived in 1989. Skrenta already had a place in history — at fifteen he wrote Elk Cloner, one of the first viruses for home computers — and later wrote the tass newsreader, which tin was built on.