Hrómundar saga Gripssonar or The Saga of Hromund Gripsson is a legendary saga from Iceland. The original version has been lost, but its content has been preserved in the rímur of Hrómundr Gripsson, known as Griplur, which were probably composed in the first half of the 14th century, but appeared in print in 1896 in Fernir forníslenzkar rímnaflokkar, edited by Finnur Jónsson. These rímur were the basis for the not very appreciated Hrómunds saga which is found in the 17th-century MS of the Arnamagnæan Manuscript Collection, AM 587b 4°, as well as thirty-eight known later manuscripts. The saga as we now have it contains a number of narrative discrepancies, which are probably the result of the scribe working from a partly illegible manuscript of the rímur.
According to Þorgils saga ok Hafliða, a saga included in the great compilation Sturlunga saga, the original version was composed by the farmer Hrólfr of Skálmarnes and was recited by him at a wedding at Reykjahólar in 1119:
Hrólfr frá Skálmarnesi sagði sögu frá Hröngviði víkíngi ok frá Óláfi Liðsmannakonungi ok haugbroti Þráins berserks ok Hrómundi Gripssyni—ok margar vísur með. En þessari sögu var skemmt Sverri konungi, ok kallaði hann slíkar lygisögur skemmtiligstar. Ok þó kunna menn at telja ættir sínar til Hrómundar Gripssonar. Þessa sögu hafði Hrólfr sjálfr saman setta.
Hrolf of Skalmarnes told a saga about Hrongvid the Viking and about Olaf King of Warriors, and breaking into Thrain’s burial mound, and Hromund Gripsson—and many verses along with it. King Sverrir found this saga amusing, and he called such “lying sagas” the most entertaining. And yet men are able to reckon their ancestry from Hromund Gripsson. Hrolf himself had put this saga together.
The saga that we now have is about Hrómundr serving king Óláf King of Warriors (Óláfr Liðsmannakonung) and Hrómund's battles with the berserker Hröngvið, as well as the undead witch-king Þráinn, a draugr (he was a former king of Gaul, Valland). Þráinn had killed 420 men, including the Swedish king Semingr, with his enchanted sword Mistletoe (Mistilteinn.) Hrómund grapples with Þráinn and wins, burns his body and takes Mistletoe. This first section of the saga corresponds with the saga described in Þorgils saga ok Hafliða. It's been questioned whether Þorgils saga could have reported such details accurately over a century after the fact, but even if no such saga was told in 1119, the basic tale must have been familiar by the mid-13th century, when Þorgils saga ok Hafliða itself was written.