Not so much a boss as a special type of zombie.  You really need to read the link I provided, it explains everything you would want to know.  Players who are on the zombie side routinely spawn in as a chosen type of "special infected" and attempt to kill the Survivors.  Realizing that the survivors have access to a limited supply of medkits and a lot more health to start out it means they are much better at staying alive than the normal zombie and honestly....even the special zombies seem to die easily.  But the fact that it takes them about an hour to escape the level is balanced against this idea and the zombies only have to suceed once to win.

Reading it now.

So do you HAVE to have four friends over, or could it be onlinr co-op?

 online co-op is crucial to this working, and I am pretty sure they already confirmed it. Also I am pretty sure the computer can take over some of the zombie slots if you don't have 8 people.  But I think you absolutely need 4 people to play survivors at least.

It's online co-op, and you can use computer players as friends as well. And this is from Turtle Rock, makers of the Counterstrike bot AI, so you know they're good with AI.

Is it PC only? Because mine is a piece of crap so it can run any games from 2007, or 06, or 05, or 04.

PC or 360.

Also, about your shadow monster, it does kinda take away some of the Hunter's originality, as he does have invisibility to partial invisibility (depending how still he is) as well.

@Ben,

True the hunter does have invisibility but his is for when he is stationary and against a wall. I can definitely see how it overlaps the uniqueness, but I think the ability to not be seen is a big one when it comes to helping instill fear. Since often the fear of the unknown can be greater than if you were to actually know what is after you. Your mind playing tricks etc...

What I am trying to say I suppose is that while invisibility is unique to the hunter currently I think if the flavor of the invisibility is unique then I think this mitigates the overlap to a great extent. And as a result it allows you to fully utilize the myriad of ways you can use invisibility to instill terror and chaos without overpowering a single infected.

Hopefully that made some amount of sense.

/sigh

I'm beginning to think the forum is filled with zombies! Nobody has a good idea? At least someone else try!

@Ben,

....seems like it will be just me and you surviving hordes of zombies....

Yeah, I've given up trying to get these guys excited about PC games. Don't worry, I've got plenty of friends who'll be buying this game on day 1...we'll fight the zombie hordes together!