chain letter that threatens bad luck
A letter or message that demands the recipient send copies to others, warning that breaking the chain will bring misfortune.
When I was in elementary school, I once received a chain letter threatening bad luck.
It's best to ignore chain letters that threaten bad luck.
A more general term for chain messages, often digital, without the specific bad-luck threat.
Compound of 不幸 (misfortune) + の (possessive particle) + 手紙 (letter). The phrase directly describes a letter associated with misfortune.