At midnight, Sirius was suddenly jolted awake from his sleep by discomfort in his right arm.
Looking down, he saw that the Dark Mark on his arm was emitting an inky green glow.
After several days of recuperation and treatment with Potions, his mind had stabilized.
"Voldemort is looking for me!" The disturbance in the Dark Mark made Sirius immediately realize that Voldemort was contacting him.
He swiftly rolled out of bed, fetched a bottle of Polyjuice Potion, and drank it. After transforming, he took on Peter Pettigrew's appearance.
He then injected magic into the Dark Mark, responding to Voldemort's summons.
Imitating Peter, Sirius said in a fawning, flattering tone, "M-Master... You've finally called for me..."
A young voice came through the Dark Mark. "Wormtail, return to my side. I need you now..."
Sirius was somewhat surprised. Why did that voice sound so young? Without dwelling on it, he pretended to be overwhelmed with excitement and replied, "Master... Can I finally return to your side... I'll come find you right away..."
The voice continued through the Dark Mark. "Very good... Come find me at 77 Uxbridge Road, London..."
Sirius replied, "Yes... Master... I'll come find you immediately..."
"Mm..." The Dark Mark dimmed.
Sirius took a roll of parchment from his pocket and wrote on it: The Dark Lord contacted me through the Dark Mark. I am now going to 77 Uxbridge Road, London.
This was an Alchemical Product Matthew had given him. There were two rolls of parchment, with the other one in Matthew's hands.
When Sirius wrote words on his parchment, the corresponding contents would appear on Matthew's parchment, and vice versa.
A moment later, the writing on the parchment vanished and was replaced by a line of words: Understood. Proceed according to plan. Be careful.
Sirius put away