>Are there any restrictions on certain race/class combinations?
>Like if I wanted to play an elven necromancer would that be
>possible?

Only certain race/class combinations are available at the time you roll your character. There's a very, very (very!) small chance that IMMs can change your race/class if you roleplay something extraodinary. But again, it is extremely rare, and not to be counted on.

>Are there organisations pitted against one another that war on
>a regular basis? What are they?

Absolutely. In general, each Cabal has at least one "enemy" cabal. A clear-cut example is Tribunal (the law-governing cabal) versus Outlander (the cabal who wants to bring down civilization).

Additionally, there are an endless number of roles you could come up with which would pit you against pretty much anything individually.