Myths of the Norsemen From the Eddas and Sagas
Page: 130That such was the shaping of Sigmund among all earthly kings,
That unhurt he handled adders and other deadly things,
And might drink unscathed of venom: but Sinfiotli was so wrought
That no sting of creeping creatures would harm his body aught.”
Sigmund now began patiently to teach Sinfiotli all that a warrior of the North should know, and the two soon became inseparable companions. One day while ranging the forest together they came to a hut, where they found two men sound asleep. Near by hung two wolf-skins, which suggested immediately that the strangers were werewolves, whom a cruel spell prevented from bearing their natural form save for a short space at a time. Prompted by curiosity, Sigmund and Sinfiotli donned the wolf-skins, and they were soon, in the guise of wolves, rushing through the forest, slaying and devouring all that came in their way.
J. C. Dollman
Such were their wolfish passions that soon they attacked each other, and after a fierce struggle Sinfiotli, the younger and weaker, fell dead. This catastrophe brought Sigmund to his senses, and he hung over his murdered companion in despair. While thus engaged he saw two weasels come out of the forest and attack each other fiercely until one lay dead. The victor then sprang into the thicket, to return with a leaf, which it laid upon its companion’s breast. Then was seen a marvellous thing, for at the touch of the magic herb the dead beast came back to life. A moment later a raven flying overhead dropped a similar leaf at Sigmund’s feet, and he, understanding that the gods wished to help him, laid it upon Sinfiotli, who was at once restored to life.
In dire fear lest they might work each other further mischief, Sigmund and Sinfiotli now crept home and patiently waited until the time of their release should come. To their great relief the skins dropped off on the ninth night, and they hastily flung them into the fire, where they were entirely consumed, and the spell was broken for ever. 
Sigmund and Sinfiotli taken by Siggeir
Sigmund now confided the story of his wrongs to Sinfiotli, who swore that, although Siggeir was his father (for neither he nor Sigmund knew the secret of his birth), he would aid him in his revenge. At nightfall, therefore, he accompanied Sigmund to the king’s hall, and they entered unseen, concealing themselves in the cellar, behind the huge vats of beer. Here they were discovered by Signy’s two youngest children, who, while playing with golden rings, which rolled into the cellar, came suddenly upon the men in ambush.
They loudly proclaimed their discovery to their father and his guests, but, before Siggeir and his men could take up arms, Signy took both children, and dragging them into the cellar bade her brother slay the little traitors. This Sigmund utterly refused to do, but Sinfiotli struck off their heads ere he turned to fight against the assailants, who were now closing in upon them.