I'd suggest checking manga forums or dedicated Bleach fan sites. They often have detailed breakdowns and recommendations on where to pick up after a specific part. Sometimes, the author or official sources might provide cues as well.
Treachery in Harry Potter fanfic could be defined as the act of betrayal by a character. For example, a supposed ally turning against the main characters like when Peter Pettigrew betrayed James and Lily Potter to Voldemort. In fanfics, it can be amplified or explored in new ways. A character might betray a wizarding school's secrets to the enemy or a friend might deceive another for personal gain, such as power or a magical artifact.
Honestly, I don't know when the TYBW manga starts. But you could search manga databases or ask in manga enthusiast communities to get the details you need.
One way to represent treachery in Harry Potter fanfic is through the use of magical objects. A character could be given a cursed object that makes them turn against their friends. For example, a necklace that slowly poisons the mind and makes the wearer do evil deeds. Also, a character's change in allegiance can be a sign of treachery. If a Gryffindor suddenly starts siding with Slytherin values and actions that oppose the good of the wizarding world, it shows treachery.