Curate, connect, and discover
Baby Harry, like 1-3 MAYBE 4. Point is, he's really young, Voldemort isn't back, and Severus had been tasked with watching over him. He constantly says he hates watching over the baby, says he doesn't want to at all, says he feels nothing for the boy.
Of course, being a spy, he has to attend death eater meetings. Per say one of the members found out about Harry, and now to "prove his loyalty" he has to bring him in. He thinks it's going to be fine, maybe a couple of harsh words, but nothing bad.
He's mistaken (of course) and has to watch as Harry is mistreated. Up to you to decide HOW mistreated. Maybe it's a couple of slaps, I prefer crucio.
Extra angst if Severus is 'invited' to join, and has to harm Harry.
Severitus but instead of Harry being scared of making the wrong move, he wants to make the wrong move. He can't stand the lack of chaos and shouting, he needs to feel unsafe because that is how he feels safe. So he keeps pushing his buttons, and he's getting punished ofc, but he's not being hit. There's no slapping, spanking, kicking, nothing.
One day, he pushes too many buttons for him to believe that he's not going to be hit, but to his surprise, he's not hit, so he loses it. He's screaming, shouting, maybe even crying a bit, asking when will he be hit because he can't handle not knowing.
Severus, calm as ever, maybe even looking down at a book or something, tells him he will never hit him because it's cruel, stupid, or wtv reason.
Cue Harry sobbing because he's never been told something like that, and Severus reassuring him.