The Silver Arrow. Ambassadors of Orealis, #1

Fourteen-year-old Finna and her twin brother Erik embark on an adventure with their best friend, Kalli and an alien creature that only Finn can see. What started as an exploration of a glacier in Iceland ends up taking them to places they could never have imagined and dangers that they never expected. Using clues from hidden papers written with a secret code, they follow the trail from the Snæfell Mountain to a cavern under the Icelandic glacier and through a mysterious vortex.
Along the way, they encounter strange and terrifying creatures, Nordic gods and an alien civilization. After Erik and Kalli are kidnapped, Finna finds she must use her archery expertise, an ancient artifact and all her courage to save them and an entire planet. The fate of the alien world and even the future of Earth depend on her true aim! Can she save them in time?