Avendar: The Crucible of Legends is a multi-user dungeon (MUD) created by Matt Wallace, Thane Williams, and Brandon Downey. It is based on a heavily modified ROM codebase (originally dubbed LOFT).
Avendar was first opened to the public in 1998 and has been in operation ever since. The Avendar Development Staff (referred to in-game as immortals) is tasked with supervising and maintaining the game environment, enforcing roleplaying requirements and developing new areas, mobiles, and items.
One of the strongest features of Avendar is its extensive player-verses-player mechanics. Unlike some other MUDs where PK is discouraged, Avendar uses inter-character combat as a method for in-game conflict resolution as well as an enhancement to roleplay. The balance of both player-driven conflict and staff-enforced roleplaying has helped Avendar remain active for nearly 10 years.
A more extensive FAQ can be found here.