The Door in the Stone by Rob King

When a mysterious woman blackmails Vic and Em into leaving our world through The Door in the Stone, the lonely siblings plunge into a war in Kavenland, a world of myth and magic. Fate leads them to meet best friends Larkin, Ariana, and Noll, who are traveling through Kavenland’s frightening forest on a quest to save their home. With unique and growing powers, the five kids must learn to trust each other while taking on the strange and dangerous creatures of the woods, including the awful Scourge, in a mad dash to save Kavenland—and each other.