Games like Avendar (there are many, though very few as long-lived or of such quality) are often called MUD
s. Avendar was constructed using free and well-known (for MUDders, anyway!) MUD software called ROM 2.4, which in turn was derived from previous software (MERC and DIKU).
This DIKU./MERC/ROM family of MUD games tends to feature very Dungeons and Dragons-style play (sometimes called “hack-n-slash”… lots of monster killing involved!). They also tend to come with the same “stock” world regions, so playing one MUD can be quite similar if not identical to playing another. One of the main aims of Avendar was to make the playing experience unique by emphasizing roleplay. There are other families of MUDs that have a strong roleplay emphasis as well, but they tend to be less combat-oriented. So Avendar is a marriage of the roleplay and hack-n-slash genres.
The biggest and greatest feature of Avendar, however, is its originality. Every step you take in Avendar is through a fantasy world that exists nowhere else. Every creature, every person, every adventure, and every quest is distinct and unique to Avendar. And with nearly a decade of development and play, the world of Avendar has become very in-depth in terms of the mythos and culture, due both to the many years of development by worldbuilders as well as the countless feats of heroism or dastardly villany from the players who have lived out their stories and left their marks.