The Portal (The Imager Chronicles)

When Denise gives Nathan a strange red stone, they are transported to the land of Fayrah where they meet Listro Q, a purple fox-like creature with a Mohawk; Astrophenix, a bear-like fellow who is bad at poetry; Samson, a flying bug with a glowing blue tail; and Mr. Hornsberry, a snooty stuffed bulldog. Nathan's selfish nature soon gets him in trouble and he is lured by the Illusionist and held prisoner in Bobok's castle. Denise and her new friends must free Nathan from his captor, and someone must make an enormous sacrifice so that the others can escape.