The name is black humour. Ever since exportation of live prisoners to Prisoner Island stopped due to a successful revolt by the Prisoners, local Governors have resorted to the quick and effective yet ghastly methods of crucifixion, cages and impaling prisoners on spiked poles. The thieves forest is a sunken valley which stinks of rotting flesh and has the largest, noisiest flock of crows to be found anyplace.
The lower valley is swampy with an old mill-house now occupied by ‘the manager’ of ‘the poles’ also called ‘the spikes’. The track now surrounded by corpses runs in two circles, one third and two thirds of the way up both hills, with several intersections. The road in and out of the valley is to either end of it following the river through. There are some old passages across the swamp but it is said to have claimed more lives than the ones you see stuck on poles and left to rot in rusty cages.
Other abandoned farming facilities include a derelict barn and a burnt out farmhouse on the far end of the valley and a tiny chapel building up a track on the hillside, which has a beautifully carved wooden door featuring an old man holding a young boy, symbol of the God of Wisdom and Learning. On the interior of the door someone has scratched a crude map of the swamp passages.
To get the best of games-mastering this location it does well to have NPCs refer to it vaguely for several sessions before the player-characters discover the thieves forest is not a thriving bandit camp in the woods but rather a grim place of death.