A Campaign of Terror - Ideas Needed

[Eldan]

So. The situation. The party has, massively and personally, pissed of a criminal gang. Their boss wants revenge.
The problem is, the party is far too high level for them to engage in anything like combat. So I've decided they will instead terrorize the party for a bit while the party tries to find them.

The party is four people, in the mid levels, but not very optimized. They don't have any good divination, especially, which should help a lot.

The other side has some 20 members, mostly warriors and rogues, mostly below 3rd level. They have a bit more money than they should perhaps have, due to activities, and some political contacts. They have a reputation for utter brutality and ruthlessness.

What can they do? I imagine that they will absolutely go after friends, allies and pets. Steal or destroy valued possessions. They may burn down buildings the party is in. But I'm trying to think of more. Both small and personal, and big and brutal, so I'm turning to you.

[Honest Tiefling]

Don't inspire terror. Inspire rage. It's far easier and cheaper, too. Burning down the building they are in is all nice and good, but you stand to lose too many valuable assets in doing so. Instead, burn down the house of anyone they love and care about. They can't protect them all, and if they do so, well, they've been locked down into being useless. Then move on to anyone who does business with them, so no one wants to associate with these people.

Of course, this could simply be buying this fine group of upstanding business-people some time. Use contacts and bribes to sully their reputation. Fires aren't following them because of coincidence, but because they're cursed, or are actually saboteurs who get angry over the littlest thing, or they use the fire to steal things. Now they're locked out of any upstanding legal work, and the common people fear them because they either believe the rumors, or believe someone with enough power is out to get them and doesn't fear collateral damage.

And if at all possible, frame them for annoying some very powerful people who don't think twice about killing those who cross them. If you only have 20 people, you can't take that sort of risk. But you can inform some cults that maybe these guys have got it in for them...Or even find actual people they've crossed and use them to soften up the group.

You can't forget to find ways to harass them in ways that prevent them from getting magical supplies or healing. Burn down a few temples, bribe a few priests, take a few hostages, but eventually, they can't recover like they used to, quite a shame.

An alternative, if the upstanding and law-abiding people are good at it, is to make the PCs afraid of a red herring with a bit of magic and trickery. Make even the party think an Archdevil of Fire is out to get them when it is really just a barrel of pitch in the right place. Use illusions, and even if the illusions are seen through, pin it on someone who has a knack for that. Obviously the group isn't afraid of you, but making them paranoid and fight shadows is always a good thing.

[atemu1234]

Have the pickpockets steal items. Have them assassinate the NPC allies, with little to no trace of who they are. Burn down their home villages, kill their families, their pet dogs if you have to.

[Gildedragon]

Fear I'd a close relative of suspense
Send them image crystals (like Polaroids) of them during intimate moments in private "secure" spaces.
Envelopes with personal trinkets of theirs or their loved ones'. "Hey Tordek where is my spellbook?" "Dunno Mialee but you got a package..." Spellbook is inside package "how'd this get here?" Spellbook has annotations and corrections, showing someone smarter than her went through it.