According to a translation of "The Nibelungenlied" that is available in the public domain, Lord Siegfried was slain by Hagen, who pierced Siegfried's heart with a spear. "The Nibelungenlied" is a Middle High German epic poem that dates back to around 1200 A.D.
The Encyclopaedia Britannica states that the German title of the epic translates to "Song of the Nibelungs" in English. The epic's author is unknown, but it is known that he was an Austrian from the Danube region. There are three main 13th-century manuscripts of the poem. The first of the three is held in Munich; the second, and most trustworthy, is in St. Gallen, Switzerland; and a third is located in Donaueschingen.