Idiomatic expression referring to a final, decisive resource held in reserve for a critical moment. Often used in the phrase 伝家の宝刀を抜く (to play one's trump card).
The negotiations were tough, but in the end I played my trump card and got them to agree to a price reduction.
treasured family sword
Original literal meaning: a prized sword passed down through a family. Now mostly encountered in historical or explanatory contexts.
This sword is a treasured family heirloom passed down in our house, protected for generations.