Um... this was supposed to be a one shot, but the reception was legendary so... here's part 2, I guess?
The trick Subaru used to get his authority of wrath was pretty rudimentary, all things considered.
He just opened a rift in space close to the location of Sirius, and then killed her with Al Karum by transfering his mana through the rift, the authority transfered to him through the rift as well. Honestly, this was something Beako could do, which was why it was rudimentary.
Of course, it was only rudimentary in the case of magic ability. He had to know the exact location of Sirius to pull this off, and he also had to find the perfect timing where she wasn't using her authority, so that he didn't get killed with her. But considering the amount of times he had repeated this first day, he knew something like this like the back of his hand.
He would've liked to use this on other Archbishops too, but all of them had either instincts good enough to dodge the attack (Gluttony) or some kind of immortality, so killing them was a bit harder, especially considering that the amount of mana he had right now was abysmal.
Which is why the first thing he did was rob stores for some magic stones. Using perfect knowledge of all the stores and its inhabitants, something like this was possible even with his weak body. He had to kill some people with his Unyielding Flames (name for wrath authority), but he had long ago gotten over the guilt of killing innocent people.
Magic stones were stockpiles of mana. If one wanted to, they could substitute the mana for a spell with magic stones.
This was exactly why Subaru needed them. Subaru could've befriended a bunch of spirits and used their mana instead, but that would take too much time.
As Subaru finished robbing his last store (the most guarded one yet), he started considering something.
'Should I... save Emilia?'
The only real benefit of saving her was having Roswaal owe him one, but that would require him appearing before her in person, which would waste time.
'I shouldn't. It's just a waste of mana and time.'
His past self would've hated him for this, but he didn't care. He had already forgotten what it felt like to love, all those memories beyond blurry. He didn't hold any feelings for anyone at this point.
"Oi! Who are you?!"
Oh, he was too absorbed in his thoughts. A mercenary who was hired to protect this shop had found him.
'I can kill him...'
But that would be a bit problematic. This was a very large store, and as such, he was planning to steal from it some more in the future.
He had specifically taken the exact amount of magic stones so that they wouldn't suspect much, but if he killed a mercenary, they would tighten their security, and breaking in later would be more costly.
'It's decided then.'
Spending the smallest bit of mana that he regenerated, he appeared in front of the mercenary within a blink. He then grabbed the mercenary's head and burned his memories.
Unyielding Flames could burn anything. Concepts, time, space and memories too.
That was the extent of his rage, his wrath. He had lost almost all emotions, but his hate for the person who cast this curse on him was unrelenting. It was unyielding. And the Authority of Wrath was proportionally strong to that hate.
This was also one of the reasons why he killed Sirius first. Out of all the Archbishops, the mad lover's authority worked for him best.
After burning the mercenary's memories away, Subaru just continued on his way.
After finding a secluded alley, he bit his finger and used his blood to draw a magic circle. After placing one magic stone on it, he activated the magic circle and started making gestures in the air.
The magic circle didn't glow, but that was because it wasn't supposed to. Subaru had no need for theatrics, so he just focused on efficiency.
After the spell was finished, he found himself in a cave full of different magical ores.
'Worked perfectly.'
Well, there was no way he would mess up a simple teleportation circle, so this was a given. The only reason he didn't teleport normally was because the amount of mana he had was limited, so even if drawing the circle only saved him one magic stone, it was still worth it.
After gathering all the magical ores nearby and some specific magical plants, Subaru once again drew a circle and teleported himself to a wide open plain with nothing in sight.
He grinded all the things he gathered together, until he ended up with a substance similar to red ink.
He used this red ink to draw a magic circle the size of ten football fields. After throwing all but two of his mana stones onto it, he activated it.
After a small amount of time, he heard a voice inside of his head.
"Thank you."
His heart clenched. Even after doing this for who knows how many times, he couldn't help but react whenever she said that.
Not long after, another foreign substance entered his body.
The jealousy, the envy inside of him manifested into hands that stole whatever he wanted.
"No need to thank me, Satella."
Using the two leftover magic stones, Subaru teleported into a misty field. Of course, this wasn't normal mist.
Subaru looked ahead and saw the White Whale, along with two small silhouettes. The two small silhuottes were Roy and Ley, of course.
When he walked closer, the Gluttonies immediately reacted to his arrival and tried to attack him, but it was useless.
Before they could reach him, both of them just helplessly fell to the ground. After which he just stomped on their heads to kill them.
What did Subaru do? Easy. He just stole their mana and stamina with Unforgettable Loss (name of envy authority).
Envy was also one of the emotions he didn't lose. How could he not envy all of those around him?
All of them could die, all of them didn't have to suffer for eternity, all of them were mortal.
Subaru was naturally envious of that. While he was trapped in a loop with no escape in sight, all of them could enjoy their life and leave footprints in the world, while all of his actions were meaningless and would be erased as soon as the loop reset.
He just could not forget his loss of mortality.
Using the mana he stole from the Gluttony twins (he couldn't get their authority until he killed Louis), he teleported to the storage of a noble named Reedback. He retrieved the Life Sword and teleported away once the alarms started ringing.
The Life Sword was one of the ten enchanted swords. This specific one allows a person to destroy souls. Subaru had once destroyed his own soul with this sword, but even that didn't end his suffering.
Using the Life Sword he easily killed both Betelguese and Capella. After that, he burned Regulus's mansion, killing all his wives in the process, and then killed him too.
He teleported to the Hall of Memories, distracted Od Laguna so that it didn't notice him and then killed Louis too.
This earned him four new authorities... but honestly, all of them couldn't compare to his authorities of Wrath and Envy. They were useful, but not nearly as much.
He didn't even bother getting the other authorities, all of them were useless to him. His pride and vainglory were non-existent, so those authorities would be beyond weak. And all melancholy did was making killing easier, which wasn't worth the effort.
"I guess I'm ready to go to earth again."
He stole some more mana from a random witch cultist while collecting the blood of all the cultists he killed.
He then teleported to random cities and collected blood from innocent civilians, while stealing their mana too.
After gathering enough, Subaru teleported to the Augria Sand Dunes and started drawing a city-wide magic circle with all the blood he collected.
By using the thousands of hands from Invisible Providence, the work was finished much faster than it should've been.
Of course, Subaru had to constantly heal himself to not die from the recoil, but it was worth it. Not like he cared about the pain anymore.
He activated the magic circle with the mana of millions, and sat down in the middle of it.
"I'm coming, earth."
He activated his Unstoppable Escape (name for lust authority), and escaped from this world.
Just a question, but do you want explanations of Subaru's authorities or not? They aren't really important for the story, but I could make a dedicated chapter for that, if you really want.