Any books are allowed. I'll add that to the first post.
And yeah, I think I might run it by my DM to see if that feat's allowed.
I think some acceptable prereqs. to that would be:
Str 15, Weapon Focus, Weapon Specialization, Greater Weapon Focus, Greater Weapon Specialization, Fighter level 12...
I'd give the two who killed the Barbarian an alignment change. Or at the very least make a note of it and see if they continue with this behavior.
From the way you described what happened they didn't panic and knew exactly what they were doing. It almost seems like they thought it was a joke...
If you didn't guess yet, I'm making a fighter and I've run out of feats. He's a level 21 human. Barbarian 1, fighter 20. So that means I have 20 feats to take care of.
My stats are (before items):
Str 22
Dex 16
Con 20
Int 16
Wis 14
Cha 16
(we rolled 3d10 drop the lowest)
So the feats I have...