solemn vow

Translations
торжественная клятва
He made a solemn vow to protect his family.
Он дал торжественную клятву защищать свою семью.
Definitions
A solemn vow is a serious and formal promise or commitment made with deep sincerity and gravity, often in a ceremonial context.
Words
solemn

adjective
торжественный
vow

noun
клятва