My group’s crew are at war with the Dimmer Sisters.
I’m a bit at a loss for ideas about how a faction with a lot of ghosts and supernatural resources on tap might conduct gang warfare.
Possessing people the PCs care about and attacking corporeally?
Telekinetic poltergeist sabotage?
Demonic terror weapons?
Please share nasty ideas about how things might go horribly sideways when going up against this faction. I feel like things need to get a bit scary.
Using spirit wardens as a weapon by sending lots of ghost to a specific area… Like the hq of your crew.
Meeting on a neutral ground for a very awkward cup of tea.
Edit: In fact, in our campaign, crew had only one meeting with 2 of the Dimmer sisters. They wanted to negotiate a neutrality pact (our Spider just learned the Ghost Contract and wanted to use it).
It started with cup of tea and talking about weather with one of the sisters. The other sat there without talking, until it came time to sign the contract. She rolled up the sleeves on her dress - imagine the Spider’s surprise when it turned out that almost every inch of her skin is covered in signs from other contracts 
Basically - they don’t have to use their resources to fight in gang war - they can make use of collected “favors”
The Dimmers could manipulate the ghost field to create echoes of the crew, revealing past crimes or sowing interparty discord, depending on characters’ histories
I like these ideas! Thank you for posting them!
I should share that Dimmer Sisters in our game are… interpreted creatively.
They do indeed inhabit a old manor house and grounds, with an ancient temple ruin and subterranean canal beneath… in the ghost field. The Dimmer Sisters’ home is anchored on the other side of a spirit well.
In present-day Duskwall, the Dimmer Sisters’ HQ is a double lightning-barriered bunker patrolled by armed hulls. Only Roslyn and occasional supply deliveries go in or out. The Dimmer Sisters are extreme recluses because they are a bodiless spirit superorganism, a braid of ghosts trailing into the past.
Part of their assets says that they have some ghost under their control. If they trail into the past maybe the sisters use the ghosts of old relatives pertinent to the crew. Then they have to fight their past and y’know…ghosts.
Alternatively maybe one of the ghosts that the sisters have on-call (so to speak) is a Luca Brasi-type or some other legendary now dead killer now bound to their service. Just some ideas! Hope it goes well!