Oh hello there. I was brainlessstorming (it's like brainstorming but less intelligent) when I had this notion.

I would like to point out I know nothing aside from synopses of what happens in the final three books, known as Harry Potter and the Remarkably Thick Doorstops. Also I admit a fondness for the more clang-bang adventures of the first four, where the herds of deux ex machina and the inconsistencies and the characters' complete inability to think outside the magical box don't matter as much.

Anyway, this uses the 'Sirius comes back from the Veil' trope, but, well...

Harry Potter and the Three Black Dogs

"Ow."

This was the general consensus of everyone in the room. Like the rest of Grimmaud Place, it was dull, dark, and generally unpleasant to be in. Even more so after the, to put it bluntly, Dark ritual that Hermione had found in the Black library and that Harry, upon learning its purpose, had become fixated on performing.

Seeing his godfather fall through the Veil in the Department of Mysteries had affected Harry greatly; learning that he'd been tricked into going there by Voldemort, then learning that Albus Dumbledore had deliberately withheld information about a prophecy concerning him, in order that he might 'enjoy his childhood', had actually turned him slightly.

The ritual was necromantic in nature, summoning someone's spirit from beyond the grave tends to be like that, but Harry no longer gave a damn. Voldemort was not going to take his godfather away from him!

They hadn't told Ron about the ritual until he'd given an Unbreakable Vow to keep his silence and assist them no matter what. Ron was their friend, but he was also under Dumbledore's thrall and would have gone straight to him. Whereupon they'd have been watched more closely than ever, and Hermione would have lost access to the library forever. That had made her even more vehement about the vow.

"Everyone will have heard that!" Ron was naturally the first to start panicking. "If they work out what we were doing, they'll..."

"They'll what?" a familiar voice asked.

"Let me guess," the same voice added, but from a slightly different position and with a smoker's burr to it. "Some n'wah had a spell go wrong."

"Sirius?" Harry's voice was faint, but it still got a chuckle.

"That's a voice I haven't heard for a while," Sirius remarked, "If I'm not dreaming, I'd almost say I'm back in –"

"– Dear old Grimmaud Place in jolly old England," rasped the other Sirius. "Now either a scrib's under my bedroll, or we should get off this poor bastard."

"Don't," Sirius spoke again, "call Sirius Pad-Foot a poor bastard." He sounded dangerously calm.

"Now I know I've gone mad," said the first Sirius, "I just heard myself with a Vvardenfell and a Skyrim accent."

Lights were still sparkling in the teenagers' eyes, but they could make out a shape too large to be Sirius Orion Black.

"Let me help you up," Sirius Pad-Foot said quietly. "Fus!"

The shape flew into three, and the shockwave blew over Ron, Harry and Hermione, causing them to slide and stagger, but now the cobwebs had cleared and they could see...

Sirius had risen to his feet. He looked somehow thin, yet tough, underneath the notably battered armour that seemed to be made of leather and something that looked like scales. A pair of odd swords, like scimitars, clinked on his hips, and a longbow peered over his right shoulder.

Sirius also picked himself up. He looked well-fed, but just as tempered, as though he had fought almost too many battles. The vest and breeches he wore were almost barbaric and clashed with the glassy green sword and the shield in the shape of a huge red diamond.

Sirius, too, picked himself up. His skin was strangely ashen, and all he wore was a breechclout and a strange, brassy gauntlet on his right hand. He flexed it, then looked around quickly before spotting a sword seemingly made of a glowing crystal in an elaborate tuning fork and diving for it.

"That's Keening," the second Sirius said in amazement, staring at the weapon. "Then that gauntlet..."

"Is Wraithguard," the near-naked Sirius confirmed.

"But Wraithguard was lost when the Nerevarine left Morrowind for Nine only knows," the armoured Sirius observed. "Not that it matters but we have company coming." So saying he turned to the door, sinewy arms folded across his chest.

"We protect the... children," said Sirius-with-the-red-shield, "Whatever attacks will rue the day it brought us here."

"Um... we did," Hermione said in a small voice.

"Tell me something only... Her... Hermione... would know," the near-naked Sirius didn't so much request as demand, just as the door flew open.

I think you can see where this is going: the magic of the Potterverse and the magic of the Elder Scrolls interact strangely, so that instead of communing with Sirius' spirit, they get Sirius himself back – in triplicate. One Nerevarine, Sirius of Redoran (probably), dead on his feet and shattered from freshly defeating Dagoth Ur. One Champion of Cyrodiil and Divine Crusader, known as Sirius the Black, who was practicing his sword work until rudely interrupted. And of course, one Dovahkiin, hight Sirius Pad-Foot. All to help the Wizarding World and one slightly addled Harry James Potter.

Naturally, hijinks ensue, with Sirius Black being seen in multiple places at once, the teaching of various (slightly incompatible) magic systems, and naturally there's some friction between all three of him and the status quo. Also, there is the question: Are Sirius back permanently, or are they only on loan...?