How do I decorate the slanted wall behind my bed?

Try wallpaper which looks like stacked stone or shiplap. That should give you a beautiful feature wall.

Does it slant from the floor to the ceiling or is it a partial slant?

You could add wall paper to give it some subtle texture. There is peel and stick kind that is easy to apply.

I think an old door or shutters make a faux headboard in the size you need would be fun. Attach to wall not bed. Depending on style and likes I’ve seen them made out of surf boards, baseball bats, water skis etc. So think textured wall art of sort. A tapestry/quilt might work too to define the space.