While the idea of an antagonist is a good one for providing some impetus to progress any major story thread forward, beyond individual shenaniganry and whatever sexual encounters might crop up, given the immortals' free-for-all survival dichotomy I'm not sure trying to shoehorn them all into working together toward defeating a single opponent would prove effective or efficient. They might work together on short-term goals for mutually satisfactory ends, but I don't see how hunting down a single immortal would be beneficial to too large a group, particularly if any members of said group aren't currently endangered by the target or believe they will be endangered by him/her in the immediate future.<br/><br/>Rather, I would suggest having the antagonistic side be the more proactive in this regard. If the idea is to have the immortals be roughly on par with one another anyway it might seem out of place to have the "evil" one be uncharacteristically stronger than the others to the point of requiring a combined effort to be killed. I'd think of it more like a serial killer: the antagonist isn't so dangerous because he's impossible to beat in combat. It's because of his methods of hunting others down. He isn't hindered by honor or ethics or rules or laws, nor is he concerned with dignity or pride. He's not looking for a fair fight. He'd much prefer to catch you in the shower, or playing with your friend's children at the park, or shooting you a few times before he takes out his sword.<br/><br/>In other words, all he cares about is<em>winning</em>.<br/><br/>I could see that perhaps driving some of the less combat-enthusiastic immortals to look for protection. Maybe not directly asking others for help, but at least moving closer to where they know others can be found, hoping to defend themselves simply by proximity. Things could snowball from there - an attack running into a third, previously uninvolved immortal, dragging him/her into the situation whether it's desired or not. Character relationships developing from there. And so on.<br/><br/>Just a thought, anyway. No idea if that's the sort of baddie you had in mind.