A reply is sent to Rivalen, it expresses in clear terms that those Shade made an attack before they were attacked and so are prisoners of war, also as a clarification the offer was to drop the attack that Ishtarland had at the time focused on the shade, such attacks were dropped, no mention of not starting a latter attack was made, not that it would have stopped us even if it had been since the Shade in question had proven their hostile intention. If Rivalen wants them returned he can be polite and they might be released after a seal has been put on their higher order magical abilities. (Note my power only has 11th level spellcasters captured.)
(Also how is Rivalen around ? To my understanding the Corrupted Allies are no more and all the Shade ever in existance are captured by the mages of the Thrillonian Alliance)