Cyberus is a monster made specifically for Legacy, a single-player campaign made for Unreal1 & ut99. In that campaign you have to destroy 4 crystals in order to kill it, because otherwise it's completely invulnerable:
GoPostal made this edit of it, so it doesn't require any external packages. Projectiles are a little different, the sinister voice it had in Legacy didn't stick with the monster, and (most importantly) this version has no crystals. I've spent a week using dozens of different weaponpacks and mutators and I've found it's nearly impossible to kill.
Been trying to find a monster for years that can't be killed... and Cyberus is it.
Enjoy · http://www.mediafire.com/file/rd5b4ikzu9k3g69/CyberusUT.zip