In his Gesta Danorum ("Deeds of the Danish People"), Saxo Grammaticus writes that Sigurd was close to his father, Ragnar Lodbrok, traveling with Ragnar in Scotland and being named sub-ruler of conquered territories. We also learn that Sigurd went with Ragnar through the Kievan Rus all the way to the Hellespont.
When Ragnar died (killed by Ælla of Northumbria, prompting Ragnar's sons to launch the Great Heathen Army), Sigurd inherited the island of Zealand, the province of Scania and Halland, the Danish islands, and Viken (the strait between Norway, Sweden, and Jutland). He is also referred to in many records as the co-ruler of Denmark with his older brother Halfdan (who was away most of the time with the Great Heathen Army).
The "evidence" of co-rulership comes from Frankish sources that mention Sigfred and Halfdan as rulers in 873 (Sigfrid and Sigurd were often mixed up in literature). The known Danish King Harthacanute (not this one in England, but an earlier designated with a "I" in Denmark) was the son of "Sigurd."
What about Sigurd's nickname? It is said he was born with a mark in his left eye that looked like a snake. You might spot the difference in the 1670 engraving pictured above.
Some of the above comes from Ragnarssona þáttr, "The Tale of Ragnar's sons." It mentions that, when word came of Ragnar's death, two of Ragnar's son, Björn Ironside and Hvitserk ("white shirt," believed to be a nickname of Halfdan) were playing tafl. That reference got me looking into tafl, a table game forms of which have been played for centuries. I'll tell you more tomorrow.
