Sirius doesnt have a master...Well if you ment Kreacher then
Sirius is Kreacher's master [Kreacher=House elf]
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Kreacher brought Mundungus to Grimmauld Place.
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Kreacher is the house elf for the Black family's home. After the
death of Sirius, he becomes Harry's to command. Harry eventually
wins Kreacher's affection.
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Is it Order of the Pheonix? If it is you need to talk to Sirius
leave the room and you will see kreacher
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He simply calls for Kreacher, who was bound to obey him.