Brutal DOOM Bug? Final DOOM: The Plutonia Experiment - Map 12: Speed - Ghost Demon
Basically an enemy with no physical presence that can't be killed by any means.
I believe it's a bug in Brutal DOOM. Brutal DOOM has a lot of bugs. I am playing on Brutal DOOM v20. Since 21 is out now, or at least coming out soon, I wouldn't be surprised if it was fixed already. But hopefully that second bug I posted isn't fixed. I like that bug...