I always found it odd that this phrase is so commonly heard in the game, especially by Cimmerians (given that we don't speak to many Nordheimers). Conan himself says it a few times, and you hear it very early in the game when you speak to Tuireach (sp) the blacksmith and Captain Redrik. Incidentally, these are quite a pair. All the Cimmerian zones are basically about the conflict between Cimmerians and northerners, Vanir in particular, yet these two have apparently been "battle-brothers" despite their people's conflict. At first, I dismissed it as simple inspiration from history - after all, the vikings (the inspiration behind the Nordheimers) settled/conquered much of the British Isles, not in the least in Ireland and Scotland whose Celtish culture is the inspiration for Cimmerians. There was even mixing of the cultures/ethnicity as Norse-Gaels/Hiberno-Norse are examples of.
This isn't really reflected in the game at all though. Kern Wolfeye has Nordheimer-features, and he's an outcast in Cimmeria who dismisses his origins. There's not a single instance of peace between the Vanir and Cimmerians that I can think of, though things get a bit murkier when you progress through the zones and see that the Aesir are merely mistrusted rather than hated, and it's a bit unclear if Aelfrit's clan are Cimmerians or not, if Atzel's bandits are Cimmerians or all Southern mercenaries etc... I haven't read most of the books yet, but it doesn't really seem to be supported there either. Crom and Ymir are gods from different pantheons, and in "The Frost Giant's Daughter", Conan is repeatedly told that he's a southerner who doesn't belong in Nordheim.
So, is "Crom and Ymir" just a weird choice for a Cimmerian oath by Funcom, or is there any support for Cimmerians giving a damn about Nordheimer gods or vice versa in the greater Conan lore?