Warning This Section Does Contain Spoilers
The handwritten note has a clue to tell us how to solve the cypher written on the note. It says "I will help with all of my six senses put forward". We are shifting all letters six letters forward in the alphabet. The cyphertext reads:
Ojih nbcm ziohxuncih qy aliq ni nby miulcha mqyyn miha zlig nby bulj iz nby xuqh um uh yhxfymm vlcabn fcabnhcha volhm mes ni nby yulnb qbcfy nby zuohu uhx zylhm qcmb nbycl nloy hugym vy bcxxyh ni uff von nby lupyh
After shifting all letters six letters forward in the alphabet we get the solved sentence that says:
Upon this foundation we grow to the soaring sweet song from the harp of the dawn as an endless bright lightning burns sky to the earth while the fauna and ferns wish their true names be hidden to all but the Raven.