You can get a general idea of how low their willpower is from checking the negative mood effect of their 'Enslaved' thought: it starts at a -15 mood penalty and rises from there.
Slaves typically won't be happy about their situation and will make repeated escape attempts until their willpower has been broken. After that your warden will fasten it to their neck, and presto! They're now a useful member of your slave-driving colony. You'll first need to make a Slave Collar at either the Smithy or Crafting Spot, then in the Prisoner Tab, select the new option, 'Enslave'. Original mod by Thirite, updated to 1.0 by Ziehn. Allows the player to take captured prisoners as slaves.
