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Sigh.... this is how people die.... You did some research... what does that mean? Surfing the internet? Which sword fern rhizome did the natives eat? What genus? What species? There are 19 species of sword ferns (Nephrolepis) in the world, according to a world expert I contacted in person. I haven't read of any having edible rhizomes. Two species do have edible swollen stolons. They do no have edible rhizomes.
However, I suspect you mean the Holly Fern, sometimes called the Western Sword Fern, which is a totally different genus and species, Polystichum, specifically Polysticum munitum. Natives roasted the rhizome. But be careful. Polystichum filix-mas is toxic, and Polystichum aleuticum is endangered.
Saying sword ferns have edible rhizomes can get one killed. One needs to be more specific.