An obsolete Scottish legal warrant
Letters of fire and sword were a Scottish legal instrument. If a criminal resisted the law and refused to answer his citation, it was considered treason in the Scottish courts; and “letters of fire and sword” were sent to the sheriff, authorising him to use either or both these instruments to apprehend the contumacious party.
Well known letters issued.
Letters of Fire and Sword were issued against the following clans:
References.
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 incorporates text from a publication now in the public domain:  Brewer's Dictionary of Phrase and Fable 1898 edition page 464 entry Fire and Sword