"This is where they live," Mad-Eye Moody said, nodding to the abandoned industrial-type building just ahead of them. "Remember your orders: we aren't here for the vampires, just the Death Eaters. Capture anyone you can get your hands on, but our main purpose here is disbanding the rendezvous. Got it?"

Sirius nodded, glancing back at the other gathered Order members, Remus and Peter and Emmeline and Benjy Fenwick. Their ranks had thinned considerably over the past few months—first it was Marlene, slaughtered while on a brief trip home to see her family, then it was the Prewett brothers, and most recently, only last week, Dorcas Meadowes had been killed by Voldemort himself. So many of them had lost their lives, so many people Sirius had teased and laughed with and grabbed after-mission beers with…and yet the ones who remained were fighting harder than ever, both for themselves and for the ones they'd lost. Sirius knew he had to be careful since he was the Potters' Secret-Keeper, but he'd never once turned down a mission he was offered. He always managed to find a way to make it out alive.

Tonight, they'd been tasked with breaking up a meeting between some Death Eaters and the leaders of Britain's largest vampire coven. So far the Death Eaters had found the vampires particularly difficult to enlist onto their side, and the Order wanted to make sure it stayed that way. Fighting vampires was not fun—even worse than werewolves, Sirius had heard.

"Stick close to me," Mad-Eye told them. "It's time for us to go in."

As he led the party forward towards the building, they each cast a Shadowing Charm over themselves to help them avoid detection. The charms were no Invisibility Cloak, but they hopefully were enough to get their team safely to the vampires.

Mad-Eye unlocked a side door and ushered them inside—they crept in to find themselves at the head of a narrow, darkened hallway, so dark that Mad-Eye permitted Sirius and Emmeline to light up their wands so they could see where they were going. Sirius held his wand up to the walls, narrowing his eyes at the dark substance splattered across them. "Is that blood?"

"Blood?" Peter gulped. "Oh, Merlin, no."

"Be quiet," Mad-Eye hissed at them. "This is a vampire den—what did you think we were going to find?"

Sirius resolutely turned away from the blood-stained walls and followed the others down the hallway until they were able to pick out voices coming from somewhere up ahead. Emmeline, the best tracker of the group, led them through the halls until they reached a door leading into an expansive, mostly-empty room—peering through the small window cut into the door, Sirius saw a group of cloaked Death Eaters clustered in its center with a group of pallid, fang-toothed vampires.

"Ready?" Mad-Eye asked. The rest of them nodded silently.

"Confringo!" Mad-Eye yelled, blasting open the door. The six of them rushed inside, wands out; "Lumos Maxima!" Mad-Eye and Benjy cried at the same time, sending two flashes of blinding light across the room. The vampires hissed in pain and scurried away, vanishing into the shadows.

The Death Eaters whipped around—there were four of them, Mulciber and Lucius Malfoy and two women Sirius didn't recognize. "Avada Kedavra!" screeched one of the women, aiming for Mad-Eye, but he and the others ducked out of the way of the jet of deadly green light.

"Stupefy!" Sirius's spell, intended for the woman, hit Mulciber in the chest instead, sending his former classmate toppling to the ground. Emmeline fell to Lucius's Body-Bind Curse at the same time as Remus thwarted him with Impedimenta.

The two sides continued to exchange spells, everything from Unforgivable Curses to Levicorpus. Sirius was preparing to Stun Lucius when his wand was yanked suddenly from his grasp, and his hands were pulled behind his back by conjured ropes; whirling around, he saw that his assailant was none other than Severus Snape, grabbing his arms and dragging him away from the others. Sirius struggled, kicking and squirming, but Snape held firm until he reached a room branching off from the main one and threw Sirius inside, shutting the door behind him.

"What the hell, Snivellus?" Sirius demanded, pulling himself to his feet as he successfully freed one of his hands from the rope. "You looking to fight me one-on-one?"

"Not this time," Snape replied. Something was off about his tone. "I'm trying to warn you."

Sirius leaned forward and punched Snape right in the nose. Snape staggered, and Sirius used the opportunity to grab his wand back and aim it right between Snape's eyes. "Warn me about what? A bunch of your Death Eater friends wanting to kill me? I was already quite aware of that, thank you very much." Sirius pulled his other hand free and wandlessly summoned a small knife between his fingers.

"No," Snape insisted; "listen to me. It's about James and Lily—they're in trouble, they're—"

Sirius whipped out his knife and held it against Snape's neck, throwing him back against a wall and pressing the blade into his skin until it drew blood. "Why would you care if they're in trouble?" he growled. "They hate you, and you hate them."

"I don't," Snape gasped; he gritted his teeth against the pain as more blood oozed from his neck. "Lily, I…I love her. You know I love her. You have to keep her safe, Sirius…please…."

And there was something in Snape's eyes, in the way his voice cracked while he begged, that made Sirius think he might be telling the truth. He did love Lily—obsessively, unrequitedly—but he loved her all the same. And if James and Lily really were in danger….

Sirius removed the knife from Snape's neck, though he kept his wand at the ready. "Tell me," he demanded.

Snape swallowed, averting his eyes. "I only just found out," he said quietly. "There's a spy in the Order, and Voldemort wants him to help him get to James and Lily—to their son, I mean. One of your Marauders. He's—" Snape trailed off, his eyes focusing on something over Sirius's shoulder. Sirius spun around, finding Peter and Remus standing in the room's open doorway; suddenly there was a pop, and Snape vanished from under Sirius's grasp. Shit.

"Who was that?" Peter wondered.

"Snape." Sirius kicked at the ground in frustration, balling up his hands into fists. "I almost had him before you two showed up."

"Sorry," Peter murmured. "We just thought you needed help."

"The rest of the Death Eaters are gone," Remus said. "Disapparated. And the vampires are nowhere to be found. Mad-Eye's ordered us to get out of here and reconvene at the manor."

"Fine." Sirius tucked his wand away and followed them out of the room, his thoughts spinning so fast his head ached. Snape could have been lying, of course, trying to sow division among the Order, but he'd looked so desperate…one of the Marauders, he'd said. One of the Marauders had turned spy for Voldemort.

He kept his eyes trained on Remus's back as they walked—he'd been acting so strange for so long, ever since they rescued him from the werewolves; what if Fenrir's pack had tempted him onto their side? What if his depression and his constant sleeping and his refusal to let Sirius near him were all meant to cover up his guilt at having betrayed his closest friends? The more Sirius thought about it, the more it made sense.

Snape was right: James and Lily really were in danger, from someone they never would have thought they'd have to worry about. One of the people they loved most in the world was plotting behind their backs to have them killed.

Sirius had to warn them before it was too late. Now, it was their turn to go behind Remus's back. The Potters needed to find themselves a new Secret-Keeper.