That is a question I do not have an answer to. Perhaps in turning they are put under some compulsion, in some way. Perhaps they are gifted a power for revenge, but must agree to answer when called. Perhaps this Wanderer genuinely considers this gift a kindness. Perhaps his dislike of the House of Ravens means he is happy to turn them to continue disturbing the peace here. All conjecture only, questions that could only be answered by further investigation.
no subject