Despite the incredible power Mark provides, there are consequences to pay, such as a drastic increase in strength with dire risks and side effects.
Demon Slayer Mark enhances the user's performance and allows them to take on stronger enemies, even Upper Moon Demons, but for a limited time.
When the Mark is activated, the user can change the course of seemingly unwinnable battles, with powers and abilities far beyond normal limits.
However, too much long-term use would be fatal. Even for El who was a Beyonder, it was still a high price to pay.
Extreme physical damage was one of them and resulted in permanent injury or even instant death. Because the body was unable to withstand the pressure of the power that suddenly increased dramatically.
"This skill is indeed extraordinarily powerful, but the risk is just as great," El muttered, feeling the burden received when the Demon Slayer Mark Skill was active. "The fee for using it is definitely no joke."
El understood, every time Demon Slayer Mark was deactivated, his body was forced beyond its limits, bearing an almost unbearable burden that damaged some of his tissues and organs.
The muscles all over his body, felt like they were stretched to the point of almost tearing, his breathing was heavy, as if his lungs were on fire. His vision seemed to be spinning, his head felt crushed.
But, when the Skill was active, a very mysterious power flowed in the pulse, and increased the value of physical attributes by 70% which made it faster, stronger, and more deadly.
"Behind the risk, this skill could be my ace in the hole," El thought. Demon Slayer Mark was the last weapon he could rely on in a life-and-death battle.
An ace that El would keep until the most critical moment, when victory or defeat depended on how far he was willing to sacrifice his body.
However, El was fully aware, using this Skill too often or for too long meant challenging death. "I have to be careful in the future, if I'm ready to use the Demon Slayer Mark Skill."
"However, in order to maximize the Demon Slayer Mark, access to the World of Invisibility is difficult to achieve in the near future, although the special effects are almost similar to Rinnegan's Eye of Insight ability."
Focusing his mind, El recalled the conditions that had to be achieved in order to access the Transparant World.
To enter the Transparant World, the user must be in a very calm state of mind, with no distractions.
A clear mind allows the user to focus completely on the small details that are often overlooked.
Mastering this mental calmness is essential to allow the user to Mark, and to be able to see the opponent's body like an intricate machine and understand how each part moves.
But, in order to access the World of Invisibility, the user of the Demon Slyar Mark needs to master the state of Selfless State or sharpening sensitivity.
The Selfless State is an esoteric and mysterious state of existence where one completely erases their Fighting Spirit, desire to fight, bloodlust, anger, hatred, malice, drive and hostility.
Once in this state, the user's behavior will become extremely calm and focused, often even compared to a plant.
Users may need to practice meditation for long periods of time, where they train themselves to completely get rid of the ego and distracting thoughts.
This could be described as a process of harsh discipline, where they learn to ignore worldly emotions and desires, focusing entirely on the battle.
Moreover, in battle, when a warrior is in a life-or-death situation, it is possible to reach the Selfless State spontaneously.
When the pressure is so intense that they go beyond their physical and mental limits, their minds can go into this state.
Among the previous two methods, there is one more method that is fairly easy for El to master.
That is by sharpening sensitivity to the environment. In this state, El must harmonize with the world around him, as if he is part of nature itself.
"Sharpening sensitivity is quite easy to do." El said without emotion, which then suddenly became more serious. "But, I prefer it when it reaches the Selfless state. This effect is still better, than sharpening sensitivity."
"I'll study the Selfess state," El muttered in reflection. "Rinnegan's influence making me lose empathy is not a big deal."
"There is a cost to every great power. There is no room for fear or doubt. This is my consideration."
"Lacking empathy is not important now. I will do what needs to be done." El said, swinging her right, closing the hologram panel.
After that, El closed her eyelids slowly, like drops of morning dew. El tried to sense the condition of her body, how badly damaged it was.
El's breathing was regular and soft, reflecting the calmness within. The air was soft like mist, blowing from her mouth.
El was fully focused on using Total Breathing Concentration, checking his physical condition thoroughly, feeling every pulse and blood flow in his body.
Suddenly El came to his senses and coughed up quite a bit of blood, "Uhuk-uhuk... Ugh, it hurts! My body feels like it's being squeezed!" El curled up on the bed, grimacing in pain.
The sheets on the bed, which had been white as a goose feather, were now stained with his blood, which was a deep red.
"Ah!" Upon entering the room, Naho was very surprised and dropped the tray, on top of which was some food for El. "I must quickly call Shinobu-sama to come over!" she said in a panic.
Naho turned around and left the infirmary. With very hasty steps, Naho went to where Shinobu was, to report what she had just seen.
Before long, Naho arrived at Shinobu's laboratory with a panicked face. "Shinobu-sama! It's serious! Adrian-sama is coughing up blood, and a lot of it!"
Shinobu's warm expression instantly turned serious. "Naho, call Aoi immediately! Tell her it's an emergency," Shinobu said firmly.
"Yes, Shinobu-sama!" Naho immediately ran to where Aoi was.
Shinobu hurried to El's treatment room. She knew very well, if El was already coughing up blood, his condition would be far from "fine" and could even be very critical.
***
El looked up at the ceiling of the treatment room with a frustrated expression. The scenery outside the window had darkened, indicating that it was already nighttime.
From morning, noon and evening, Shinobu continued to care for El who suddenly developed a very severe coughing up of blood. El didn't feel like bothering Shinobu, "What would she like? For an apology?"
For a while, his coughing up blood had stopped, as Shinobu gave his an injection of anti-inflammatory and antibiotic medication.
When coughing up blood, El's esophagus was like swallowing a hot nail.
His lungs felt like they were being squeezed.
His organs seemed to be moved from where they were supposed to be.
It was a horrible sensation.
El sighed heavily. "Damn... Why is every MC in fanfics always overpowered? Whether it's genius, talented, or worse, the MC is an Uchiha descendant with the power of Zafkiel Kurumi. Even then, he's loved by the world? This is cheating!"
(N/A: Nyahahaha, let's make you suffer in the Pain! I really like seeing you suffer. I wonder what will be good after this? Do you have any ideas, Reader?)
El closed his eyes, trying to remember his conversation with Tanjiro before losing consciousness. "Looks like, my guess was wrong again this time. It's not just a few weeks... it might take a few months to fully recover."
Although El relied on his VIT attribute value of 37 attribute points and added the effects of the Desert Worm Pants equipment, which could restore health 1:10 every 20 minutes, his physical damage rate was much faster.