[Anduia]

Originally it was the TMI-2 mudlib running on MudOS [1], though my understanding is that they heavily modified the codebase over time. The library was never officially released as open source [2], but the code (in C) is included in a ZIP file alongside installers and related files [3].

As for the JavaScript client, it appears to be proprietary.

[1] https://t2tmud.org/boards/news/1402345353.php

[2] https://en.wikipedia.org/wiki/LPMud#TMI_Mudlib

[3] https://mudbytes.net/files/view/1041