noun
missive
MISS-iv/ˈmɪsɪv/
A letter or other written message.
A long handwritten missive lies beside an envelope and a fountain pen.

Etymology
A missive was a letter characterized as sent, especially by superior authority. The adjective became a noun for the message itself, first including official commands and later letters more generally.
In context
the asares additionally had a missive to bring to the commander of Fleet ctis the enemy approached in numbers
More examples
- The director sends a formal missive explaining the new rules.
- His latest missive fills three pages with complaints about parking.
- She keeps her grandfather's wartime missives in a wooden box.
Forms
missive · missives