Seltani is an online environment for creating multiplayer, choice-based textual worlds. It was created by Andrew Plotkin.
Seltani is based on the Tworld engine (open-source). Tworld can be considered as an attempt to create a MUD with a Twine-style interface. The environment is hypertext rather than the traditional command-line MUD. New areas are constructed in a wiki interface, using text marked up with a Python-like scripting language.
The Seltani server itself is an (unofficial) Myst fan project. The Tworld engine is customized to follow the setting of Myst Online -- worlds are Ages, the hub is the ruined D'ni cavern, portals are linking books, and so on.