I will skip the part of the dialogue with God because I know you are tired of seeing the same scene.
-----------------------------------------------------
The snow fell amidst a forest covering the ground, standing there was a man in simple clothes with white hair and red eyes, his steady breathing hid his intensity as he saw the blue floating panel in front of him.
"I really reincarnated, it wasn't a dream!" Mark said to himself smiling as he remembered the conversation he had with God moments before, the superior being had told him that he had died unjustly and would receive a system and reincarnate but only now did he see how happy he was.
Without hesitation, he gave a mental command to activate his system and the blue screen in front of him changed.
(Multiversal Ability System!
Sending relevant information to the host's mind in 3, 2, 1!)
At that moment, Mark felt a slight tingling in his head and a large amount of information entered his mind, "It's not how I imagined it but it's not bad." I said to myself going over the information.
In summary, the system, when I meet certain conditions, would give me the option to choose an ability or a random set from a specific universe, but don't be fooled, choosing an ability doesn't give me the rights to use it.
I couldn't just choose an ability from Goku now and expect to use it because my current physical body wouldn't withstand the pressure and I wouldn't have the energy to make the ability work, the system only shows me how to make it work, achieving such requirements dependent on me.
(Ding!
Congratulations for completing your reincarnation, you have received the right to choose one ability from the following 3 universes.
1- Baki
2- Jujutsu Kaisen
3- Avatar
"Some very good choices." I said to myself and began to consider, currently I knew I was in the world of Demon Slayer and as far as I knew this world had no energy techniques to develop any kind of energy, "Although it's a pity, I choose the Baki universe." I stated.
(Ding! You have received the Baki Martial Arts Set!
Transferring knowledge.)
The next instant I felt my mind go blank, a large amount of information gushed into my head making me feel cold and have difficulty standing, but even so, a smile formed on my face.
Thud.
The next instant, Mark's body collapsed onto the snow.
Several hours passed and when it was beginning to get dark, two figures passing through the forest saw Mark lying on the ground, "Gramps, look, there's someone unconscious." A boy with yellow hair spoke and soon the two approached.
These two were Zenitsu and his Master.
"He seems to be unconscious, carry him Zenitsu, let's take him to the mansion." He spoke and soon the two left.
---------------------
Hours later.
Mark let out groans of pain when he finally opened his eyes, his head hurt and throbbed but he forced himself to sit up, he was lying on some kind of blanket on a wooden floor, the night had already fallen outside making the world almost completely dark.
"You're awake." I heard a voice and looked to the side and found an old man whom I immediately recognized as Zenitsu's master, he seemed to recognize the surprise in my gaze and laughed, "Why are you so surprised?"
"Ah, I'm sorry, it's just that you remind me of someone I knew." I said, lowering my gaze, and countless thoughts ran through my head, this was an opportunity to learn the Thunder Breathing techniques and I wasn't going to let it pass, "Unfortunately, he was killed by demons, like my family." I said, faking a sad smile.
The air fell silent for a while and the old man stopped smiling, "My condolences, boy." He said and stood up, "Rest for today, we will talk more tomorrow." He said, giving me space, and left, closing the room door.
Soon, a few days passed and I became familiar with the old man Kuwajima, Zenitsu, and Kaigaku. Just like in the anime, Kaigaku was a hopeless jerk, but I avoided trying to conflict with him. As for Zenitsu, although whiny and a bit annoying, he was quite friendly and we soon got along well.
It took a bit of work, but I managed to convince the old man to teach me Thunder Breathing. It wasn't easy because he didn't expect to accept more disciples in this life, but he decided to give me a chance, and so I began the brutal training.
Long runs in the morning, endless push-ups and squats, carrying a bucket of water up and down the mountain. I did everything with the utmost determination I could muster, and as soon as my physique began to improve, I also started training my martial arts.
I could feel my body being remolded with every kick, with every breath, it was exhausting but at the same time rewarding. "Hey Mark, why do you train so much?" Almost a month and a half later while we were training, Zenitsu asked me.
I took a deep breath, stopping my push-ups, and looked at him. "Because I want to be stronger. The only way to protect yourself and others in this world is by becoming stronger." I told him, and I saw him smile as he looked at the sky.
"I wish I had your determination. If I did, I could be as strong as Kaigaku and make Gramps proud of me." He said, and we were silent for a while. "The master is already proud of you. What you should seek is to honor his legacy." I said to Zenitsu and soon went back to doing push-ups.
He looked at me for a while and soon also started exercising again, this time in silence. For the first time, Zenitsu felt something change inside him, but he would only discover it later.