This is all from memory so, I might be missing some things:
Adds 10 extra weapons
Changes the damage ratio of all weapons
Imports the "gravity mines" from one of the Q2 expansion packs
Adds togglable "moon boots" (low gravity, on a per-player basis)
Adds a grappling hook, with swing physics
Has "secret" command to turn invisible. (I can't remember what it is). Because back in 1999 I didn't realize people could unpack binaries and read them.
My cousin and I burned hours in Quake2 with a mod that did these. Jumping around in low G while grappling around all over the map while trying to hit each other was fantastic - thank you for some of the best early FPS gaming I can remember!
https://xn0.co/Xeno-Mod_1.9_Setup.exe
Because I'm sure everyone on HN just downloads execs from a random internet source and runs them!
But if you have the OG QII, are curious, it is safe to run.