Shadow of Yserbius is a graphical MUD originally published by Sierra Online.
The goal of the game was to kill an evil elemental creature called En-li-Kil. There were a number of "rooms", where up to 30 (later 60) people could meet and adventure together in groups of up to four. Combat was turn-based.
The available professions were Barbarian, Knight, Ranger, Thief, Cleric, and Wizard. Player races included Human, Orc, Elf, Troll, Dwarf, Gnome, Halfling, and Gremlin.
Offline play is also possible
Source: Wikipedia, "Shadow of Yserbius", available under the CC-BY-SA License.