Alright, this happened often enough during my research for me to make a post specifically about it.
Meet Trevorcard
Wandering off into the regions man was not meant to see, too far astray from the path of light, will not go unpunished. Trevorcard is a spirit that will haunt you, even after your return. He posseses the soul of your former companion, Trevor, and knows two forms, one that is relatively sane to the human eye, if still eldritch, and one that is far more unworldly, distorting his very surroundings frequently.
His main attack is that of a chain, though he is prevented from actually connecting it. Instead, he will freeze motionless, should he attempt to use it regardless. Other weapons cannot be used either, with the sole exception of a measily stopwatch if it is used in such a way it won't trigger his chains.
In his humanoid form, he posseses the ability to shapeshift into a flying creature - one that is foreign to the human comprehension, but acts much like a Bat does.
Furthermore, the way his tormented soul seems to take damage is random. Sometimes, his entire life force may be depleted in one strike against him, at other times, he will endure a blow absolutely unharmed.
There are conflicting reports of his cries of agony when he is hit - some have described it as a big rumble or crash - whereas others have likened it to the sound of a knife being thrown. Everyone will agree though - its sound is very eerie.
If only he was useful in an actual TAS.