The monster was not wrong on Sirius's life being shit as it currently was with him about to fight a being of immense strength. Sirius was honestly shitting some very thick bricks right now.
That's when Sirius was launched to the side via the fist implanting into his face that he failed to react to. Sirius was facing an opponent stronger than when Scathatch held back a considerable amount of her strength.
Sirius was still only a few months into what is supposed to be a five year training montage and he is already fighting someone his master might have some fun against. Though knowing her and her prowess, she might not.
'If my reinforced body couldn't react, how am I supposed to fight this thing!?'
Sirius activated thought partition and acceleration coming up with ideas as fast as he could while he put his focus on fighting the monster.
"Ahahahaha! This is so much fun! You actually haven't died yet and are still fighting back, but for how long can you keep going, for how long can you entertain me?!" The monster grinned manically.
His kick broke several of Sirius's ribs and pierced one of his lungs, leaving it hard to breath. As he lay on the ground wheezing, he was hoisted up by his neck face to face with the monster.
"You are tenacious I'll give you that. But you will die in the end and end up in my stomach."
Sirius grinned, his bloody teeth shining through. "That so? Well my little friend says otherwise." Then he shoved his automail hand deep inside of the monster and snapped his fingers, making the body glow a bright orange as it heat up.
"Fuck you, bitch!" He kicked off the stunned monster and made a barrier of earth and metal, shielding him from the explosion of fire. However he had a feeling that wouldn't be the end of it and sprung out of his defenses.
All of his reinforcement magecraft went into the most optimal parts of his body. His nerves, essential muscles, and bones. The prana that was being spread evenly before was instead being put at max capacity into parts of his body.
Now he could react to the punch and defend without having his bones shatter on impact. He swung his sword decapitating him yet again only for the flying head to come back like it has an extension cord of a spine.
"Alright. The fight was fun while it lasted, but I am hungry and you are my prey, so farewell." The monster's hand shot out, it's nails sharpening only for it to phase through Sirius.
"Huh?"
Sirius snickered. His true magic was honestly not that great with his current understanding. He had to understand a certain amount of the thing he was trying to hide from before he could hide from it. Take Scathatchs spear for example.
He had the understanding that it was a spear and what it was made to do allowing him to hide from that part. But he didn't understand the part about the spear breaking causality itself to kill its target.
That's why it didn't work. Of course he didn't find that out until after the battle but with his ongoing battle he was able to scan the monster with each physical confrontation and understand him. Thus, true magic is effective.
Then while the monster was stunned he used his displacement magecraft to displace the monsters body parts, in different areas of the Pit.
One after the other the monster disappeared until the only thing left was the heart that still thumped with ferocity. However before Sirius could loot the heart for experimentation, blood pooled around the heart, covering and outlining the monsters form once again.
'Where the hell is he getting this energy for his regenerative properties?! He should have run out of prana by now to allow his abilities to function!'
Sirius couldn't understand until he thought back to the monsters earlier words. The words that told Sirius that the monster could see the souls and devour said souls, which shouldn't be possible. Unless the souls aren't devoured and are merely contained in the body.
The fedora wearing man dusted off his suit before turning to Sirius. "That was a nifty little trick you did there. Mind telling me how it was done?"
Sirius was still in thought acceleration, when asked so he ignored it. He had the best bet with displacement itself as alchemy messes with matter and energy. He could displace the monsters soul into a object and place a pseudo personality in the body.
This would allow him to trap and kill the monster while getting him the body to study.
"Not going to answer me boy?" The monster kicked Sirius right in the abdominals, sending him flying into a rock wall. "Where's the respect for your elders-" Before he could finish, space bent separating his head from his torso once again.
"Sigh, boy haven't you figured it out by now that I am immortal. It useless~" Right as he attached him self several spear of rock stabbed into him, locking him in place for Sirius to grasp his head. The monster looked on arrogantly before his instincts went off like a morning alarm.
'What-' He couldn't even finish his thought as his perspective changed to him looking at his body once again. Only this time, it was whole with not a single scratch on him. 'What the fuck happened?!'
"That's what you get for being arrogant you piece of shit!" Sirius slung the body over his shoulder before he displaced back to his workshop on fumes. He had to hurry as once the rock doll he put the monsters consciousness in was destroyed or the monsters on will overpowered the spell, he would get his body back.
First he sealed the body and doll in a reoccurring loop that switched the consciousness between the two so fast he couldn't do anything fast enough to escape his constraints that were powered by the monsters own energy.
"I will have your fucking soul, you bastard!" The monsters voice filtered between the two. Sirius used this chance to convert the monsters energy into a alchemical transfiguration spell. Prana flared as the monsters body changed on a fundamental level into a metal cylinder.
He would convert this into a philosophers stone, but with the monsters will still in the body, thanks to it being so strong, the stone wouldn't even listen to his commands and allow him to use his developing magecraft.
However he can still use the cylinder as a spectator and bring other undead monsters to his workshop to be devoured and watch the phenomena to copy the process.
"Soon, soon I will have my very own magecraft. I will be able to create life itself....." Sirius didn't know how wrong he was, blinded by the goal he had set out to do. Reviving his family.