Nexus War is a series of free, online, browser-based MMORPGs inspired by, and very similar to, Urban Dead. The original Nexus War was created in open beta by UD player Jorm on May 22, 2006, with game concepts and ideas steadily being tweaked or added over the following years. It was officially closed in October 2009, following a final celebration of the game. Following the shutdown, many Nexus War players migrated to Nexus Clash, a sequel created by Nexus War site admin BobGeneric that continues the Nexus story.
The main differences between Urban Dead and Nexus War include a thematic change from survivors and zombies to angels and demons, a vastly expanded class and skill system, the addition of magic and morality, a much smaller and more heterogeneous game world, the addition of actual geography, and a greater degree of flexibility and capability in most other elements of the game (e.g. groups, items, communication, etc.).
For additional information regarding the original Nexus War, please see the official website.