This is the famous portals.wad converted to UDMF format, designed to (eventually) work in all 3 major ports (ZDoom, Eternity, and 3DGE). While I don't think it will work in 3DGE right this moment (and Eternity will refuse to load it), I do think it can be used to test at least one of 3DGE's basic features.
I obtained permission from esselfortium before converting this map.
Here's a video of this map in action: https://www.youtube.com/watch?v=3W0YyvGlr5k
Credits:
esselfortium - Original portals.wad designed to work in Eternity
Marisa Heit + Christoph Oelckers - "dumpmap" ccmd from older versions of ZDoom, used to convert the map to Hexen format
Pascal van der Heiden + MaxED + contributors - GZDoomBuilder - which was used to convert the map to UDMF format
(As a note, this version contains additional void sectors which are designed for ZDoom compatibility)