Horse of Bells, by Pelaam MM

Prince Donal and his brother Caolan are sent to safety when their step-mother Queen Doireann is suspected of an attempt on Caolan's life. Caolan doesn't want to break his pact with the mysterious stranger from the forest who rescued him however his loyalty lies with his brother and they depart.

When recalled home Donal is manipulated by Doireann into undertaking an honour-bound quest to obtain the Horse of Bells from the Dark Prince. Failure means forfeiting his right to the throne. Further, if caught he faces death for attempting to steal the Dark Prince's magical horse.

Caolan accompanies Donal on his quest and they are joined by the enigmatic Traveller.

But is he friend or foe?