It would turn scary if the atmosphere would become as dense as the core of a star. Then too, a lot of stuff depends on the type of star we’re talking about. Are we talking about some wimpy red dwarf core density? Yeah, we’ll fuse the hydrogen in the atmosphere/on the surface all right. U wanna fuse nitrogen? Oof. U’r gonna need a much bigger star than that.
So basically, to become scary, ur atmosphere would have to be a lot more dense than what it would be if it was fkin solid (like if the gases were literally solid). It wouldn’t thus be an atmosphere.