According to the Penguin English Dictionary,
Peradventure 1) adv archaic perhaps or possibly 2) noun archaic doubt or chance
I don't know if anyone uses the word 'peradventure' anymore. The dictionary notes it as archaic. However, I came across it twice in two days in completely separate works. The first was The Legend of Sleepy Hollow by Washington Irving. He uses the word several times in his fun, if overly long, tale.
One day later, I saw the word again, this time in a quote from a 17th century work concerning snakes and dragons.
You read archaic works, you see archaic words, fair enough. I think in all cases, I saw it used as an adverb. Having read the definition, I'm not real sure how to use it as a noun.