I find Greebo to be an odd being. He was gracious enough to entertain my questions about his background, as Forsaken tend to have unusual and interesting stories regarding their lives, and their afterlives. Usually those who have lived that long have wisdom to impart, and I thought it best to pick his brain, so to speak, on the past of the Grim in his experience.
The fact that he had never been human to begin with was a complete surprise. I had always been under the assumption that Forsaken were all once humans, though then that begs the question about why other human-like races couldn't become Forsaken. Apparently, he began as an experiment, his being yanked from a sort of void-like existence and shoved into a newly created, forcefully evicted Forsaken. From then on he gladly served Sylvanas, the leader of the Forsaken.
At this point in time, the Grim was young, reckless, and impetuous. The Horde was under the command of Thrall, who was not happy with the Grim because of their targetting and antagonizing the Alliance. Sylvanas did not seem to mind, but she was wise enough to observe the growing guild. In that capacity, Greebo was sent to spy on the Grim.
The first setback he faced was when the original soul belonging to the body he inhabited came back and repossessed him. Without a host, he was forced to inhabit a living human, and he found his eyes opened to how much the Humans hated the Forsaken. I think this may have played a part in how he came to lead a guild whose motto is Peace through Annihilation.
The second setback that faced him was a blessing in disguise, or at least I'd have seen it that way. The Grim launched an attack on Stormwind, and in the ensuing battle his human host as well as his original body were slain. Seizing the opportunity, he reclaimed his initial body, and before he'd fully regained awareness he petitioned the Grim to join their ranks.
He says the Grim has matured in the time that he's joined- the recklessness isn't as strong, and the members are more devoted to the cause. Like a finely crafted weapon, the Grim is able to direct their force where need be.
I had more to ask him, but his attention was called elsewhere. Somehow, I can see the guild starting as he claimed, and with the right guidance becoming something to truly be feared.