The Magistrate is a high official of the xenophobic Confederation of Earth and husband of that timeline's President Seven of Nine.
In the alternate timeline Q creates, humanity has formed the Confederation of Earth, a regime that conquers or wipes out alien species. The Magistrate is one of its leaders and is married to the Confederation's version of Seven of Nine, who serves as President. He presides over the public execution of the last Borg Queen, which Picard, as the Confederation's celebrated general, is expected to carry out.
In Penance, the Magistrate grows suspicious as Picard and Seven stall the execution, and he beams aboard La Sirena with security officers, shooting Elnor. In Assimilation, Seven distracts him long enough for the crew to overpower and kill him.