From last week's announcement, "What is Tesseract? Tesseract is a sort of Frankenstein's monster that escaped the lab when I ripped out the static lightmapped heart of Sauerbraten, squished it under-foot with extreme prejudice, and stitched it back together with deferred shading and shadowmapping."
Cube 2 Tesseract provides deferred shading support, HDR rendering with tone-mapping and bloom, omni-directional point-light shadowmaps, sunlight cascaded shadow-maps, screen-space ambient occlusion, screen-space water reflections, refractive alpha cubes, and refractive glass material.
Right now this advanced "Cube 2" code is being housed on GitHub.
This improved engine code is still available as open-source under the Zlib license and "the codebase is still in a state of high flux, so I would expect a lot of changes coming in the future."
The main developer says Sauerbraten / Cube 2 development will continue but this is "just a fork, a what-if experiment to see what it might look like if it dropped all concerns about compatibility and good performance on old hardware and just tried to modernize." Additionally, "Tesseract is not at this moment endeavoring to be a stand-alone game, but just another parallel codebase people interested in the engine can choose from to work from if they want to mod it into a game of their own."
Embedded below are some Tesseract demo videos.