Bandit war? Or a mysterious challenger?

[RavenStarver]

So my party recently finished off the last campaign arc, killed the bandit lord, took his keep for themselves, and passed fourth level.
I'm trying to decide on what to do for the next session, we've already discussed that they'll have about a month or two of downtime in-game as they consolidate their power base and handle the politics of the Eastern Territory that arise when you suddenly own your own fiefdom.
There was one group of bandits they haven't handled that kind of retreated to their hold in the mountains, and they still have a sizable force. So they could always come out in force and we could have a fun single session battle of Pelenor Fields type deal.
Or I can move straight into my next plot arc which, unfortunately I don't want to spoil in case any of my players frequent these forums. But it involves a sizable threat from the south that'd launch an attack against them or on people nearby.
For reference, the party is all fourth level, a Wolfen Knight who's sworn to a family that's sworn to Llorn, a Coyle Thief who's sworn to himself but plays nice, a Priest of Apis, and a pyromaniac Mind Mage Kobold (I know kobolds normally don't get psionics but I let him because I don't really care) they've had a little parade of various allies and NPCs that have come and gone and have endeared themselves quite a bit to the locals. Saving a town from pirates, ending the threat of a united bandit army, and being fairly standup the whole time.

[RavenStarver]

Oh I forgot to mention, the Knight has spent a fair portion of the game either press ganging bandits into his mercenary army, or recruiting for his mercenary army, and currently has about 120 fighting men and women.

[Soldier of Od]

I would have the last bandit force attack them first, as part of the 'consolidating' their territory as you say.

UNLESS this mysterious force from the south might be able to make a deal with these bandits and join forces or manipulate them into doing some of their dirty work. Just as the heroes think they've got the southern threat on the ropes, a surprise attack by the mountain bandits they had all but forgotten about might be fun!

[RavenStarver]

I would have the last bandit force attack them first, as part of the 'consolidating' their territory as you say.

UNLESS this mysterious force from the south might be able to make a deal with these bandits and join forces or manipulate them into doing some of their dirty work. Just as the heroes think they've got the southern threat on the ropes, a surprise attack by the mountain bandits they had all but forgotten about might be fun!

They might, the threat from the south is going to be a slow building threat I hope, the party is reasonably powerful and have crushed any physical threat I've thrown their way so I'm going for more cloak and dagger this time around. Don't let them pin down the guys

[RavenStarver]

So after some checking around it appears that none of my players frequent these forums at all, so spoilers can abound!
Essentially I've repurposed the Tri-Fang adventure to take place in the Eastern Territory. So far they have recovered the artifacts and delivered them to the villains without realizing what's going on.
The downside to all this that leads to why I'm pondering what to do is that the cult as is written in the book, would not be a threat to the party they'd steamroll the guys in a fight. Hence why I'm pondering over what to do. One idea I've tossed around in my head is to vastly increase the number of cultists and have a Thulsa Doom and his snake cult style group.
Large numbers of the cult moving about and stirring up trouble between groups so they can harvest the materials they need for their plan.