This piece of fan art is a complete fiction as Zenitsu has never become a demon as depicted in the original artwork, but what it does is imagine a world in which that would happen in this way and shows fans the product of such an event.
Zenitsu is a powerful Demon Slayer and a member of the Demon Slayer Corps. Although he was originally roped into joining the Corps by his mentor Jigoro Kuwajima, Zenitsu later realizes the importance of the Demon Slayer Corps.
Unfortunately, the girl betrayed Zenitsu and ran away with the money. Then the debt collectors came and asked Zenitsu. to return the money. At that moment, Jigoro came and rescued Zenitsu from that situation. Jigoro told Zenitsu to become a Demon Slayer.
Fortunately, Zenitsu somehow survives Daki's attack but struggles a lot being trapped beneath the ruins of the fallen house.
Although seen to be a cowardly and anxious person, he possesses a split personality that when unconscious makes him one of the most powerful thunder breathing style demon slayer corps member who has taken on and defeated many demons. Zenitsu is known for his unique personality and comes off as a weak-willed individual.
Demon Slayers are known for their strength and bravery in battle, but not Zenitsu. Despite his cowardice, he's one of the strongest in the series.
Zenitsu doesn't realize his own strength
In fact, he's upset — he was hoping he would die and get out of having to fight demons. He's not at all confident in himself, often repeating that he "knows he'll die soon" and "it's only a matter of time." Fortunately for him, he does have amazing powers.
This mastery allowed Zenitsu to form a Seventh Form of Thunder Breathing that is unique only to him: Honoikazuchi no Kami. While this move might seem similar to Thunder Breathing's First Form, the speed and power of this technique are on a whole new level.
Zenitsu Agatsuma is a part of the Demon Slayer Corps who the previous Thunder Hashira trained. Zenitsu is a skilled and able member of the organization but gets easily scared when facing demons. But as the series continues, we watch him grow as he eventually overcomes his fear of demons and fights them head-on.
After his treachery, Kaigaku later becomes Zenitsu's archenemy. His cowardice would later lead him to his downfall when Zenitsu overcomes his insecurities as Kaigaku mocks Jigoro. Kaigaku's Thunder Breathing training would lead to his Blood Demon Arts being related to lightning.
When Zenitsu sees her for the first time and discovers that she is a cute girl, he quickly gets mad at Tanjiro because he thinks that Nezuko is actually his girlfriend instead of his sister.
After Muzan was defeated, all the demons he created also perished immediately, so there was no point of a Demon Slayer corps anymore. As a result, Zenitsu did not get to become a Hashira even after becoming strong enough to bie one.
Unfortunately, Gyutaro spots Inosuke and stabs him right through where his heart should be with a sickle imbued with deadly poison.
Working together, both Demon Slayers managed to fend off the strongest demon to have ever lived who was casually fighting them, though Tanjiro ends up losing an eye and is severely poisoned as a result, nearly causing him to lose his life.
Tanjiro Is Strong, and Stronger than Zenitsu as he uses Sun Breathing.
Thunder Clap and Flash—Godspeed is the most powerful of Zenitsu's derivations of the Thunder Breathing first-form technique.
There are six forms, but Zenitsu could only learn the first form said to be the basis for the others yet he honed it to where he could fight against several strong demons. He later created a seventh form and defeated his former senior who had become the Upper Rank Six of the Twelve Kizuki.
This breathing style originally had six forms, each being extremely difficult. Zenitsu mastered the first form to its full capacity but failed to learn the rest. However, the first form itself is extremely difficult. With his master gone, Zenitsu can no longer learn the remaining five forms.
Nezuko Kamado
Zenitsu also protects Nezuko multiple times when she is in danger, evident during Zenitsu's first meeting with Inosuke Hashibira and the battle against Enmu in the train. Eventually Zenitsu and Nezuko would marry and start a family as evidenced by their descendants.
Zenitsu then is slashed by Kaigaku with the Thunder Breathing, Second Form: Rice Spirit.
Breath of the Sun
It's the oldest and most powerful breathing style that was used to develop the Flame, Water, Moon, Thunder, Stone, and Wind Breathing techniques that demon slayers use to this day. It mimics the intensity and versatility of the sun, lessening a demon's ability to regenerate.
The eccentric and flamboyant Tengen Uzui is the current Sound Hashira. This Breathing Technique gives him echolocation and other advanced hearing abilities.
But, comparing the two characters in their best forms, Zenitsu might be just a tad bit faster owing to his Godspeed move.
During Tanjiro and the rest of the Hashiras' showdown with the Demon King Muzan, Tanjiro is mortally wounded and dies. At the same time, Muzan receives a particularly fatal lil' booboo himself. As Muzan is about to shuffle off the mortal coil, he uses the last of his powers to turn Tanjiro into a demon.
The reason for this is pretty simple. Zenitsu wasn't able to master the other 5 forms. He never wanted to learn any breathing techniques. or fighting techniques, nor he wanted to join the Demon Slayer Corps. He was forced, not in a bad way, to train because his master, Jigger Okuwajima, saw potential in him.