(non-con) Severus is raped and blackmailed by Harry, but is too humiliated to report it to Albus,

thinking Albus wouldn't believe him. After all, Albus didn't listen to him when James Potter humiliated in fifth year either.

Perhaps Harry snapped because he blamed Severus for Sirius' death or some other reason, but continued doing it because the sight of Snape, normally prideful and scathing, writhing pathetically on his cock was deeply arousing to him.

Bonus if (switch) he also binds Severus at some point and rides Severus as Severus does not consent.

Perhaps Severus eventually lets Albus read his mind through Legilimency. Albus may suspect the memories were tampered with, but Severus says "This is precisely why I didn't tell you..."

(But the memories were tampered with. Snape took out any hints at all that Snape took pleasure from being fucked by Potter.)

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