In the beginning of Season 5, they can haunt a person and torture them with images of bad things that person has done in the past. This makes them go mad with guilt.
In "Lost in the Shadows," Carl mentions that he is constantly seeing images of all the bad things he has done in his past. Melinda figures out that the Shadows have gotten to him. They can corrupt people and control what they do and say. Not only can they control people, they can also control ghosts. Their power of control and torture is threatened by Melinda and her son, Aiden. Since, they have the gift to crossover earthbound spirits into the light, the Shadows become weaker. The shadow also have the ability to make people feel colder and possibly ghosts too because ghost children call them "the cold things".
In "Lost in the Shadows" the shadows tried to feed on a little girl's ghost but she was saved by Aiden and on the episode "Old Sins Cast Long Shadows" the shadows fed on the ghost of an old lady, Madame Greta, because she failed the shadows. She was ripped apart into four pieces and each piece must have been turned into shadows. In both episodes "Lost in the Shadows" and "Old Sins Cast Long Shadows" the shadows were shown to be weakened by even the light of a flashlight. They also put fear into the hearts of all their servants by promising to kill them or someone close to them.
This happened to Bedford. His mother died, but that might not have been the Shadows' doing. It could have been natural. The Shadows did kill Bedford when they found out the that the Book of Changes erased itself so they could not read it. Bedford had spent a long time working for them and trying to find it. He tried to explain that it wasn't his fault that it was erased. But they were so angry that they made a bomb go off and killed Bedford. In the case of ghosts they promise to feed on them if they don't follow their orders. This happened to Madame Greta when she let Cassidy, Naomi, and the male ghost go into the light when the Shadows were about to take them. It almost happened to Julia in "Lost in the Shadows" but Aiden saved her by scaring them away with a flashlight.
The Shadows are seen creeping into Aiden's bedroom while he is sleeping. But, a light is seen and the Shadows run away. This light is thought to be from the Shinies, who must have been protecting Aiden. (Aiden is the only one who can see both Shadows and Shinies). After the Shinies scare the Shadows away, the Shadows go into Melinda's bedroom and take control of her. They make her act extremely angry, especially toward Aiden, who is a threat to them.
During the last episode of the entire series, Carl, the Watcher, tells Aiden that he needs to find the Shinies and get them to be brave enough to scare the Shadows out of Melinda. So Aiden gets Jim, Delia, Ned, and Eli to take him to the shop, where Melinda is. After Melinda says to Aiden that she is not his mother, he agrees and tells her (the Shadows) to let her go. She refuses, so Aiden walks to the center of the Square, and tells his friend Cassidy (a Shiny) to find the others and bring them to him. Cassidy explains that they are all too afraid of the Shadows. But Aiden encourages that she and the others are stronger than the Shadows. So Cassidy, takes his hand and begins to glow, making her stronger. She puts out her other hand and a chain of Shiny children holding each other's hands form around the Square, and creates a beautiful light. They all walk toward the shop and Melinda becomes free from the Shadows' control. She lets Aiden know that he is her hero and they go off to the hospital to help a ghost crossover.