Noun
an officer (as of a legislature or court) who maintains order and executes commands
Source: WordNetBlack Rod also holds the office of Serjeant-at-Arms of the House of Lords, and in this capacity attends upon the Lord Speaker. Source: Internet
The Gentleman Usher of the Black Rod's duties may be delegated to the Yeoman Usher of the Black Rod or to the Assistant Serjeant-at-Arms. Source: Internet