The window isn't locked, more stuck-like when Achak tries it. It's already broken, but in order to fit through, you'll need to break it the rest of the way.
Achak's low-light + flashlight show a 1.5ish meter drop and then water. It appears as though the entire basement is flooded. From what you can see, the basement is a rectangle, and the window leads in at about the halfway mark. In the center of the room, there are a number of old washing machines that have been pulled together, and there is some sort of planking -- maybe old park bench seats? -- set between them with 3 generators on top. The wiring is held to the ceiling support beams with what looks to be wire hangers, and the lines run to the far upper-right corner of the basement, where it looks like there is either another room or a staircase. Nothing is moving in the basement.
Sister Rebecca's thermographic vision picks up the generators, but nothing else seems to be warm down there. Nothing seems astrally active either.