This take place at dusk time when nagisa is coming back home on his way back he hears a chainsaw noise right in the dark alley nagisa was afraid who's in there and don't want to look. But his curiousity got the best of him as he drew closer he saw a strange looking living chainsaw? Nagisa was shock after what he saw. But it appears to be harmless as he drew closer he said "its okay lil guy i won't hurt you" the chainsaw sound then stop and the chainsaw looking dogs start to hapily bark. Feeling empathy to the creature he decided to take it back home his mom however said otherwise she was extremely pissed after what nagisa just bring

But nagisa beg his mother not to take the chainsaw dog away out of small consent that she had she allowed one pet and so the bound begins. Right after they arrive in his room nagisa said "aren't a strange looking boy! You must be lonely out there?" The dog bark happily in respond nagisa pause for a sec and said " do you understand what I'm saying" the dog and nods nagisa smile and say "aren't you a just a very smart dog!" Before bedtime comes nagisa was eating dinner and he saw the dog was eating more human food then an avarage dog but he couldn't care less. At least he was happy in the morning nagisa give the dog a walk and come a cross a dark alley and he hear a noise a devil appears and its a needle devil nagisa and the dog he now call "tomu" got brutallly attack nagisa with only human level assassin experience can't keep up with the devil speed nagisa then was persumed dead until tomu give nagisa a second chance in life in order to achieve his goal to kill koro sensei this is his contract to save the earth from destruction tomu fused his heart to nagisa making him regenerate all of his before nagisa waking up notice tomu is missing until he saw a pull cord on his chest he was grieving after he thought tomu sarcafice himself to save him the needle devil look back feeling in rage this human is still alive nagisa think to himself and look at the pull cord not knowing what it does. he pull it out of pure desperate

As he pull rhe cord he start to feel the pain in middle face and the chainsaw come out

Nagisa then transform into the chainsaw devil this give him a unique abily such as super strength speed. Durability and stamina (what he dosen't know he has the ability to devour and erase devil)

After a short pain is now become a wrath of rage he then begun is assault on the needle devil

(To be continued)