How about making them as city encounters, you may meet some shadow thieves, and some boys and girls (you can't kill the kids of course) on street, and they will pick you, your party members, and practically everyone in the area blind. You can try back-pickpocketing them before they run off. And they are especially abundant in the Docks & Bridge Districts.I suggested an idea like this quite some time ago in a different forum - I was considering writing one.. BUT it would just be irritating the way you have it described.How about a pickpocket who basically robbed the PC and his/her party blind.
Think three different theives... if you catch him you get to kill him.
They would have to rob NPCs as well as the party.
One 'good' (cautious) theif, one 'bad' theif that will steal from everyone in range, and the third I never decided on.
Stealing items is iffy - what if you steal a quest item.
Leaving the map's no good unless you reloacte them to the next map and let the PCs chase them around the city.
Everything you lost to them, you must buy back at Renal's Shadow Thief Headquarter, or one of the Black Market thieves.
And if you join Bodhi's Spellhold route, you get to clear the Shadow Thieves for good and receive an extra reputation point.