It's not Frieden (pl.) but rather Frieden (sg.).
Friede is a masculine weak noun that (theoretically) takes -n in all endings except nominative singular... but for some reason, it can take -n in the nominative singular as well, and indeed I would say that Frieden is now the most common base form, with der Friede being less common.
So the nominative singular has two possible forms.
(An interesting related word is Drache, which is also Drachen in the other forms... and that has also formed a secondary nominative singular Drachen with -n, but there the two words have received different meanings: der Drache is "the dragon" (mythical flying beast) while der Drachen is "the kite" (flying object out of paper or plastic). In all other cases, they are identical, e.g. den Drachen could be either "the dragon" or "the kite".)