Faction to whom you especially hate to lose (fluff-based)

I always feel bad about fighting the Protectorate with Ret. I mean, come on guys, we both hate magic. You help us kill the heretic mages, then we kill the rest of your impure mages when that's done. See, good deal! You get to kill heretics, we get to kill mages. It's good!

Also Ret v Grymkin just because why? The elves aren't corrupted by Menoth, they aren't summoning any Infernals. Leave them alone mang.

gameplay wise: Hate losing to Legion since thats my faction and i allways feel like i should have played better 
Lore wise: Circle. Still not sure if i understand their fluff right, but from what i gather these guys wanna end the world, or at least bring back the wurm, who is then gonna end the world. Doesnt seem very nice to mee

I'm pretty sure that's the exact opposite of what they're trying to do? Unless the fluff has changed significantly since MkII, the general idea was the the ley lines of the world are the source of the Devourer Wurm's power or something, but civilization chokes up the ley lines. Right now (as in, for all of recorded history) the Wurm is distracted by it's battle again Menoth in Urcaen, but if it starts to notice that it's power is weakening, it will come back to Caen to try to figure out why. When that happens it will basically wipe out all (or at least most of?) humanity.

Or at least that's what they believe (this kind of fluff, like most, is presented to us in the form of dialogue and writings from in-world characters, meaning it's what they believe but not necessarily the "actual truth" of the Iron Kingdoms world). So to prevent (or at least delay) the Wurm from destroying humanity, the Circle has been subtly slowing the growth of civilization. I believe that lately they've been more actively, and more overtly so, as the subtle and secretive methods aren't cutting it anymore due to how much civilization has grown and how much destruction of nature is occurring.


Edit: Ninja'd!

There is divine magic in the IK, or at least there was when it was a d20 game. The fluff still references old Menite Priest-Kings performing miracles but it was a pretty rare thing. These days I'm not sure there is much of a distinction, though there might have been the early MkI books. 

We still see some things that are definitely divine magic, like what happened to Severious when he died, but I don't think any of the Protectorate warcasters are strictly 'divine' magic in the classic DnD sense. The modern IKRPG doesn't handle them any differently. 

On the elf side we know priests do (did) have a direct magical connection to their specific god since the Rivening happened. All of a sudden one day every priest except those serving Scyrah (and Nyssor but none of them lived in Ios anymore so most of the elves didn't know that) suddenly went crazy and started attacking people and killing themselves because they lost their connection to their patron god. Characters like Harbinger and the Avatar prove that Menoth is in direct communication with at least some members of the faith, though whether he's the only reason Harbinger has magic or if he just picked someone who already had the warcaster talent I don't know. 

Oh, you're very likely correct. The Protectorate totally engages in tons of heresy in the name of the Crusade, though the creation of the Avatar implies that Menoth is okay with it (unless it's not actually Menoth who had them build it, which is my pet theory with absolutely no evidence except that I think it would be a cool twist). 50-60 years ago every single noble Protectorate warcaster would have been burned at the stake by the scrutators.

They bound the souls of several Vassals to it, that are giving it focus? That would also explain why it never bonds to a warcaster, because if someone did, they'd find out