The shella (it's not capitalized) is a "spectral wendigo" with a form of possession. It's detailed in the writeup for Corey Martin in the old Prime Runners sourcebook (pp. 86 - 88). Basically, it can turn an already rather tough badass in a really tough badass with Regeneration and enhanced senses. Oh, and an endless craving for metahuman flesh.
As that book took place in 2055, I doubt that ol' Corey is still with us, but the shella (or it's pals) may still be around and they're hungry. Oh yes, they're hungry.