Place a lightstrip on the windowsill and cover lightly with fake cobwebs, which will diffuse the light underneath. Then, attach Halloween silhouettes to the window: a bunch of flying bats or a witch riding a broom, for example. The lightstrip will cast light upwards, illuminating the display.
Bright idea: Bring sound into your Halloween decorations with Philips Hue + Spotify! Create a new Entertainment area with your Halloween lights (including your window lights), and then tap the Sync tab in the Philips Hue app. Play any Halloween song or sound effect, such as thunder or ghostly noises, and your lights will react to the sounds.