Into Shadow - The Tallan Chronicles

Muireann spent six months planning to kill the dragon that killed her family. Now the dragon is dead, and she's realized that it was actually the lesser threat guarding a devastating weapon that, if it falls into almost anyone's hands, will bring war and destruction to her world. It can't be left where it is. It can't be hidden. It can't be trusted in the hands of anyone in power or who seeks power. Muireann's only option becomes a reluctant quest with the elf she rescued from the dragon, a would-be knight, a selkie, and an elven mage to find out as much as she can about the weapon and seek a way to understand the magic that created it. Pursued by a dragon-worshipping cult bent on revenge and a hidden enemy that will do anything to remake the world, the five friends must try to find answers before all is lost. Muireann thought that killing a dragon was going to be impossible. Saving the world is going to be much harder.