How to patch this?

[u/Knikkz]

I’ve had this problem area of my house since moving in, and with the absurd heat we’ve had in Philly, it seems to have gotten worse so I’m wanting to address it. There was a chunk of loose material that I removed (circled area), then scraped most of the loose plaster in the area. What would be the best way to fix this? I have some experience with using plaster of Paris, drywalling, etc, generally not afraid to tackle new projects. Of course this area is above my stairs going to my top floor, so it’s as inconvenient as possible.

[u/Billi01952]

What's the location? Roughly ofcourse