A Luduan is a Light Fae species that seeks and demands the truth.
Although Luduan do not seem to "feed" on humans or other Fae, they do have an insatiable desire to know the truth. If a Luduan asks a question and it is not answered truthfully, they will do whatever they can to discover the truthful answer.
In Destiny’s Child, Dao-Ming agreed to help Trick remember what had been blocked from his memory on the condition that he spoke the truth in response to her questions.
Not only do Luduan have the ability to detect what is truth versus lie, they are able to force humans and Fae to tell the truth (even what they are not consciously aware of) with the clicking of their long nails and a hand gesture.
In Chinese mythology, a "luduan" was a beast which could detect truth. Rulers such as the Qianlong Emperor would surround his throne with luduan in order to properly subdue his subjects. The luduan were incense burners.