Plot basically:
This was around the time when Zenitsu met Tanjiro and Inosuke also Nezuko lol
One day Zenitsu got sent on a mission by himself while there a demon attacked him and planned to kill him.
The demon thought Zenitsu was dead and went away. Somehow by luck Zenitsu survived the attack but was now a demon-
He felt sad as he knew he couldn't go back to the demon slayer corps as he was now a demon. Not knowing what to do he suddenly saw Kaigaku his former senior. Kaigaku wants Zenitsu to forget about his friends and focus on his power as he is stronger now then when a human. Possibly even in the future become a 12 kizuki. Zenitsu reluctantly agrees but on one condition that he can have his memory erased by someone. Kaigaku then takes Zenitsu.

Another thing this is another AU- That I will work on (I'm still working on if Kaigaku was a good demon series dw)
Also in this AU Gyutaro and Daki are still the main uppermoon 6 and Kaigaku is still a replacement, but he just has uppermoon 6 in his eye as he technically is.