Truth Chains are magic collars that can strangle a liar to death. It is never called this but rather called a magic or golden lasso. Agad placed one on the neck of Thronis, and unable to remove it, he is bound to never lie again.
While not being able to remove it, Thronis has learnt how to recreate them. He placed some on the Knights of the Dawn in order for them to give him the Precious Figurines when they raided the Dragon Temple.
Source[]
- Secrets of the Dragon Sanctuary (Fablehaven #4)