Sword of Freyr
In Norse mythology, the sword belonging to Freyr, a Norse god associated with sunshine, summer and fair weather. Freyr's sword is depicted in Norse mythology as one of the few weapons that is capable of fighting on its own. After Freyr gave up the sword to Skírnir for the hand of the giantess Gerðr, he will die at Ragnarök because he didn't have his sword, fighting Surtr with an antler.
Freyr asks Skírnir to bring Gerðr to him, Skírnir demands his sword from him, and Freyr readily gives it. The loss of Freyr's sword has consequences. According to the Prose Edda, Freyr had to fight Beli without his sword and slew him with an antler. But the result at Ragnarök, the end of the world, will be much more serious. Freyr is fated to fight the fire-giant Surtr and since he does not have his sword he will be defeated.
In Ragnarok the sun shines "from the sword of the gods". The idea is that the sword which Surtr uses to slay Freyr with is his, "the sword of the gods" which Freyr had earlier bargained away for Gerðr. This would add a further layer of tragedy to the myth. Sigurður Nordal argued for this view but the possibility represented by Ursula Dronke's translation that it is a simple coincidence is equally possible.
Appearances in Popular Media and Culture
In Rick Riordan's Magnus Chase and the Gods of Asgard, the sword of Freyr possessed by the protagonist is called Sumarbrandr meaning 'summer sword'.