dark secret

Translations
темный секрет
He kept a dark secret from his past hidden.
Он скрывал темный секрет из своего прошлого.
Definitions
A dark secret is a piece of information, often about someone's past or actions, that is deliberately kept hidden because it is shameful, morally wrong, or could cause harm or scandal if revealed.
Words
dark

adjective
тёмный
secret

noun
секрет