Both my kids went through a stage like that. We sat with them on their bed until they fell asleep. It took ages some nights.
I've read about using "monster spray" too, which is just water, maybe some lavender oil in a spray bottle, and let them spray it once or twice. I wouldn't mention monsters though, just say it makes the scary go away.
We also let our kids still fall asleep with a lamp on in their rooms.