Complex sentence: Outside he is attacked by an ex-Nibelung who lost his immortality for trying to take all the treasure, it is also Hagen's father Alberich, he soon defeats Alberich and takes his tarn-helm, an item that lets him take the shape of anyone else.
Split into simpler sentences: Outside he is attacked by an ex-Nibelung who lost his immortality for trying to take all the treasure, who happens to be Hagen's father Alberich. Erik soon defeats Alberich and takes his tarn-helm, an item that lets him take the shape of anyone else.