Sirius's son

Occulmency

As it turned out, Kreacher had been lurking in the attic. Sirius said he had found him up there, covered in dust, no doubt looking for more relics of the Black family to hide in his cupboard. Still, it wasn't that convincing though, but it makes Sirius satisfied. And it seemed that after spending all that time up in the attic, it had made Kreacher be in a better mood on his reappearance, his bitter muttering had subsided somewhat and he submitted to orders more docilely than usual.

As the date of their departure back to Hogwarts drew nearer, Harry became more and more prone to what Mrs. Weasley called 'fits of the sullen', in which he would become taciturn and grumpy, often withdrawing to his room for hours at a time. His gloom seeped through the house, oozing under doorways like some noxious gas so that all of them became infected by it.

But, on the last day of the holidays, when they were in Ron's bedroom, Harry and Ron were playing wizard chess and watched by Rigel, Hermione, Eliza, Ginny, and Crookshanks. Mrs. Weasley poking her head in, said: "Harry, Rigel, Eliza dear, could you come down to the kitchen? Professor Snape would like a word with you."

Rigel looked at Eliza while lying upside down, Harry did not immediately register what she had said; one of his castles was engaged in a violent tussle with a pawn of Ron's and he was egging it on enthusiastically.

"Squash him- squash him, he's only a pawn, you idiot. Sorry, Mrs. Weasley, what did you say?"

"Professor Snape, dear. In the kitchen. He'd like a word."

Harry's mouth fell open in horror while his twin looked confused. He looked around at Ron, Hermione, and Ginny, all of whom were gaping back at him while Rigel just shrugged at him. Crookshanks, whom Hermione had been restraining with difficulty for the past quarter of an hour, leaped gleefully onto the board and set the pieces running for cover, squealing at the top of their voices.

"Snape?" said Harry blankly.

"Professor Snape, dear," said Mrs. Weasley reprovingly. "Now come on, quickly, he says he can't stay long."

"What's he want with you?" said Ron, looking unnerved as Mrs.Weasley withdrew from the room. "You haven't done anything, have you?"

"No!" Rigel indignantly while the twins shook her head.

He pushed open the kitchen door a minute or two later to find Sirius, and Snape seated at the long kitchen table, where Sirius and Snape were glaring in opposite directions. The silence between them was heavy with mutual dislike. A letter lay open on the table in front of Sirius.

"Sit down." Said Snape as they shuffled into the kitchen, Eliza sat down next to Rigel wo sat to his father's right.

"You know," said Sirius loudly, leaning back on his rear chair legs and speaking to the ceiling, "I think I'd prefer it if you didn't give orders here, Snape. It's my house, you see."

An ugly flush suffused Snape's pallid face. Harry sat down in a chair beside Sirius, facing Snape across the table.

"I was supposed to see you two alone," said Snape to Harry and Eliza, the familiar sneer curling his mouth, "but Black —"

"I'm their godfather," said Sirius, louder than ever.

"I am here on Dumbledore's orders," said Snape, whose voice, by contrast, was becoming more and more quietly waspish, "First for your son, Professor McGonagall has sent these books for you."

Rigel looked confused but nodded. The book McGongall sent was an Advanced Guide to Alterations which was mostly about human transfiguration. 

"The Headmaster has also sent me to tell you both, that it is his wish for you to study Occlumency this term."