This is pretty typical but no fun for her. But try hard not to let her trips to your bed frazzle you. This seems like it takes forever when it's going on but it will pass, eventually.
Be careful to take her seriously and never dismiss what she's saying -- I do not think you are doing that now, I'm just noting that it's so easy for us adults to get frustrated and say, "Now that's enough, this is silly" when we're worn out with a child who can't settle. She needs the security of knowing you and dad take her fears seriously whether they're rational or not. Fear, at her age, can't be rationalized away. It sounds like you do listen to her and she does communicate about this with you,, which is good -- she is willing to talk about what scares her and that's what you want her to feel comfortable doing.
Try working with her to create her own ritual to banish the shadow. Get a new, clean spray bottle and stickers of things she really likes, positive things like favorite characters or sayings or letters to spell her name, etc. Then have her fill the bottle with water you've set aside beforehand (so she doesn't see that it's just tap water) and she can add one drop of food coloring in her favorite "magical" color. This is now "dream spray" that dispels any bad things in her room and makes the air full of good dreams. Let her spray it high in the air in her bedroom each night as she says whatever special spell she wants -- she can shout loudly, "Good dreams come, bad things be gone!" or she can whisper or sing or whatever -- just make it a ritual and tell her she has control over making her room and her bedtime better. She needs to feel some control here. If she still sees the shadow despite dream spray etc., tell her that it takes time and to try again that night.
Have you walked over every inch of the house by day with her to ask her where exactly she's seen the shadow? Try it, and be sure to find something interesting, nice and distracting in each room. Note that "there's nothing here in the dark that isnt' here in the light" -- though she may not buy that for a while yet.
Also have you sat in her room at bedtime, with the lighting exactly as she would have it after "lights out" time, and looked very carefully to see if there are shadows being thrown by objects that you, as the adult, just don't notice much -- but which may frighten her? A blanket that lies over the back of a chair could look like nothing by day but cast a scary, person-sized shadow in the glow of a night-light or -- since she doesn't want any light -- there can still be shadows in a room, cast by light from outside at night. Look all around while you sit with her and pick out each shadow and identify what casts it. If something seems odd, get up and show her that you can move the object and the shadow changes or goes away. You might also need to just remove things that cast odd shadows.
If she really needs to sleep with you for a time, let her. Do put her to bed in her own bed, with a ritual, maybe an extended but consistent bedtime if stories distract her. But letting her into your bed some of the time, even many nights, for a while at this time does not mean that she will still be in your bed in a year. Truly. And it will not reinforce her fears, or mean you're "caving in" to her, if you let her take comfort with you and dad at times. If you need instead to stay in her room while she goes to sleep in her own bed, again, that does not mean she will forever need you to do it. She really will get past this and it's not spoiling her to comfort her, as long as you also give her some control over the fear as well, and work with her on feeling like she can banish the shadow.
If you just don't want to sit in her room while she goes to sleep, try sitting outside her door with the door cracked so she can see you but you are not right inside the room. I did that for some months as a transition when my daughter was learning to sleep in her toddler bed and it really helped keep her in bed --she knew I was there but also knew I was not going to be inside the room. She didn't come out to me or pop up and after a short time I could leave my post. Just an alternative to consider.