__ Letters, Anonymous Abusive Correspondence
Puzzle 5 of Valuable Things Hard Pack
Question is: __ Letters, Anonymous Abusive Correspondence
and the answer is: POISONPEN
Word Meaning:
POISON PEN — (idiomatic) A manner, means, or tone of expression had by a written work, characterized by spitefulness, defamation, and/or intimidation directed toward a person, organization, or point of view.
Definitions from Wiktionary.