It's the language used for some types of magic, religious texts. Things that are important are usually in the old languages or demonic ones.

Between the two camps?

One is the romantic soulmates like everyone thinks about when you talk about soulmates. There other is someone who shares part of your self. Two bodies, one soul. Nothing except death separating the pair. [At least that's how it's supposed to work even if according to all Alec's research it rarely does.]