The thralls consume blood packs to survive.

To unposses press square or x on console:p

I've found that putting a Thrall into any of its helper modes and then leaving the area, they immediately stop working. Coming back you have to cryo them and then set them to it again before they start. This is the same for logging off and back on. This has to be a bug, because otherwise they're more headache than they are worth.

Thralls can gather resources for you if you make them