The final final demo before Sonic: Lock & Load version 1.4.
Includes most things that'll be in the final v1.4 release. I say most things because I can't stop adding gameplay features (which is why this is Final Final and not just Final).
You will obviously need a DOOM II IWAD or Freedoom: Phase 2 to play this. Just drag and drop your IWAD in the folder if GZDoom doesn't autodetect it.
If you're playing on macOS or Linux, just open Sonic-LockandLoad-v1.4-demo.pk3 with your GZDoom installation. Which I hope you have. You need GZDoom, by the way.
This is magnificent