La espada del tiempo (Magnus Chase y los dioses de Asgard 1)

The Norse gods have awakened.
Magnus holds the secret that can stop the end of the world.
But that same secret could also destroy him.
Magnus Chase has always been a troubled kid, living alone on the streets of Boston, dodging both police and thieves. One day, a new shadow joins his pursuers: his mysterious and until then unknown uncle Randolph, who reveals an impossible secret to him: Magnus is actually the son of a Norse god. The Viking myths are real. The gods of Asgard are preparing for the Great War. Trolls, giants, and horrendous creatures are training for Ragnarok, the end of the world.
With the help of Sam, a Valkyrie who secretly hates him, and the dwarf Blitzen and the elf Hearthstone, both fugitives, Magnus will travel through the nine worlds in search of the only weapon that can stop Ragnarok: a legendary sword lost thousands of years ago... And along the way he will have to make the most difficult of decisions. Because sometimes, to save oneself, it is necessary to die.