The duel between the three Marauders and Lucius Malfoy started simple enough. Sirius was able to use some Black Family spells to block Lucius' attack. However, things got a whole lot worse real fast. Lucius was not being possessed at this time, but he was using dark magic to battle their spells. The basilisk had been ordered to kill them, so they were having to dodge the large snake while keeping their eyes closed and blocking incoming dark spells.
Sirius was just wondering whether or not he should send up red sparks when he heard another hissing that he knew didn't come from the basilisk.
The gaboon viper decided to have a conversation with the basilisk. "Why are you trying to kill the students?" the viper hissed.
The basilisk stopped in its tracks. "Another speaker?" the basilisk asked.
"Yes, now please answer my question," the viper replied.
"I have been ordered to kill any Muggle-born student I could find, and I am now being ordered to kill two pure-bloods and one half-blood. So far, I have been unsuccessful."
"Who is ordering you to kill these people?" the gaboon viper asked.
"That older boy was possessed by the Heir of Slytherin, Tom Riddle," the basilisk answered.
"How?" the viper asked angrily.
"There's a book that boy has," the basilisk answered. "Now, if you'll excuse me, I have people to kill."
"Do not kill them," the viper ordered.
"I don't listen to anyone except the Heir of Slytherin. Now, stop delaying me," the basilisk hissed angrily. The viper knew what he had to do. He dug one of his large fangs into the basilisk's head. The basilisk hissed in pain as the poisonous venom took effect. It took moments for the basilisk to hiss its last words.
"I hate you, speaker," the basilisk said. With that, the basilisk lay dead on the blood-coated ground.
Sirius knew something was up. He heard a lot of hissing back and forth, and while he couldn't understand what was being said, he didn't have time to think about it. McGonagall had joined the duel.
"Mr. Malfoy, explain yourself at once!" McGonagall demanded.
"I didn't do anything," Lucius drawled.
"Professor, he was being possessed by that book over there," James pointed out. "He is not currently being possessed though. Even when he's not being possessed, he's still a jerk. He has admitted that Voldemort has tasked him with getting rid of every Muggle-born student."
McGonagall was more angry than they could remember her ever being, and that was saying something.
"You will not be getting away with this, Mr. Malfoy," McGonagall said angrily.
"Oh, yes I will, Professor," Lucius replied coldly. "Because I will be killing you tonight, so you can't punish me." He then started shooting curses at the older witch, but she was able to block them all.
The duel was impressive, to say the least. McGonagall was very good. She was able to block all of Malfoy's spells, and send a few of her own. After ten minutes however, she was starting to slow down. A particular horrible cutting curse hit her shoulder.
"Professor!" the three Marauders exclaimed.
McGonagall didn't even seem to hear them. She was still trying to send spells at Malfoy even as she swayed on the spot.
The next thing they knew, a powerful wind spell caused Lucius to lose his balance and smash into the wall. Everyone looked behind them to see an outraged Arcturus Black with his wand out.
"That should keep him out cold for a while," he said, moving to McGonagall's side. "We need to act fast." He then called out, "Zippy!"
"Yes, Master Black?" Zippy asked, popping into the chamber.
"Get Lucretia. Bring her here. We need her to change into her phoenix form quickly. A teacher is severely injured."
Zippy's eyes widened in surprise. "Yes, Master Black. Zippy will be getting Mistress Lucretia right away!" The house-elf popped out of the chamber.
"Aunt Lucretia can turn into a phoenix?" Sirius asked, shocked.
"Yes, I can," Lucretia said from behind him. She took one look at McGonagall and immediately turned into her phoenix form. She flew over to her shoulder and cried. Before their eyes, her shoulder healed. There was not even a scar.
"Wow!" James said, impressed. "We should learn how to change into animals too!"
Lucretia turned back into her human form. "Father, she will need to go to the Hospital Wing, but she will survive," she reassured Arcturus. "Also, that book over there, it is clearly covered in black magic. And no, I don't mean black as in our family."
"I could tell that there was some kind of dark magic in that book, but I haven't gotten close enough to realize it is black magic," Arcturus said thoughtfully.
"Black magic?" Severus asked.
"Black magic is magic that is forbidden, a kind of magic that you three should not even know about," Arcturus told them. "It is the worst kind of magic. Lucretia, please get Professor McGonagall back up to the school and into the Hospital Wing. Then please come back and get us." Lucretia nodded, and turned back into her phoenix form. She flashed out of the chamber with McGonagall.
"The magical signature on this book is from Tom Riddle," James announced. "It even says it on the cover. I think it's his diary."
Arcturus nodded. "I have a theory," he said, leaning down to the dead basilisk and retrieving one of its fangs. He stabbed the diary with the fang. There was a scream, followed by a black mist. Once the mist disappeared, Arcturus turned to see Sirius looking horrified, and the other two looked confused.
"That wasn't… was it?" Sirius asked weakly.
"It was," Arcturus confirmed. Sirius felt like he was going to be sick. James and Severus were still confused, however.
"I'm missing something," James said.
"You don't want to know," Arcturus said, making it clear he was not about to tell them anything. Lucretia came back at that moment. The four of them held onto her as she flashed them back into the headmaster's office.
"Are any of you injured?" Arcturus asked.
"No, Headmaster," they all responded.
"Good. Sirius, please stay behind. You two, please head back to Gryffindor Tower."
James and Severus left. They didn't know what Sirius and Arcturus had discovered, but whatever it was, it was bad. They knew that they would be told when the time was right.