The decryption challenge's Fortbyte number 38 is waiting for you on the most northerly sky platform, just next to Lazy Lagoon. Be careful, you will have to wear the "Vendetta" skin to be able to collect the Fortbyte.
This outfit is available at level 100 of the paid version of the battle pass.
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The challenges and Fortbyte locations of the Fortnite Season 9 decryption challenge, as well as the complete list of Fortbytes, are to be found in this article.
