(SoB fluff) Looking for some help as to who they should be up against in the fluff.

Basically, they inhabit the desert world of Pyrandris. It's a world they were stationed on over thirty years ago.
They have barracks built, along with shrines and churches dedicated to the worship of the emperor as well as to the Living Saint, Fairen.

Although I can't decide on the main conflict in it. I was thinking either being invaded by Tyranids. Or having it be a necron tomb world where the necrons laid dormant until not long ago.

So basically a toss up between Nyds and Necrons.

Afterwards, the Sisters can't stand up to their foe anymore and are slowly pushed back to their last bastion with only an Exorcist and not even five hundred other sisters left.
Then, as they were about to lose hope their Living Saint awoke from her meditation deep inside one of the chapel catacombs, and helps turn the tide of war.

Fluffwise I see Sisters against Necrons more often than Nids.

Either will work though.

I suppose an easy way to decide it is which models you would prefer o paint/have available to decorate bases with.

So nyds XD I have enough nyds to pile an inch high on a 24X24 inch display board. ALL THE NYDS!!!

And if they're up against Necrons more then I'll do Nyds.

If memory serves, the first encounter the Imperium had with necrons was when they eradicated an Sob outpost. So with necrons there already are some bad blood between them and sisters.
That could serve for some background, motivation and decision making for the sisters

Jehoel wrote:

If memory serves, the first encounter the Imperium had with necrons was when they eradicated an Sob outpost. So with necrons there already are some bad blood between them and sisters.
That could serve for some background, motivation and decision making for the sisters

A friend was telling me about this too. If you want to do nids, one of the Ciaphas Cain novels has them attacking a world on which some Sisters have a chapel. That might provide some nice inspiration for dioramas, combat scenes and such.

I've just read a bit about mutants and Ordo Hereticus in the Warhammer Wiki.
So SoB are the military arm of the Ordo Hereticus, one of whose main targets is mutants.
Genestealers are like Tyranid/Human mutants.
This could cause the Battlesisters to get actively involved of their own accord with Nids who are originally Ordo Xenos business.
Could also make for some fine petty Inquisition infighting.