The Dark Master is the main antagonist of the 2007 coming-of-age fantasy film, Bridge To Terabithia, based on the novel of the same name. He is a dark figure made up in by Jess Aarons and Leslie Burke use in their adventures in Terabithia. He represents the issues Jess has with his father, and arguably his grief after Leslie's death.
When two outcasts, Jess Aarons and Leslie Burke first befriend each other, they find a secluded area in the woods, they decide to name the place "Terabithia", where they hang out to take a break from reality. In their adventures within Terabithia, they act as the king and queen of the fantastical kingdom. In their adventures, they make up the Dark Master as the villain, and spend their time battling his minions and trying to free his slaves. His presence is always detected by the sound of chains rattling.
As their adventures go on, it becomes apparent that Jess has used the Dark Master as means of dealing with his father. His father was often hard on Jess, and had very little time for him because his family was poor and Jess was the only boy in a house full of sisters. This connection is never revealed until the end.
At one point, the Dark Master has summoned a troll in Terabithia, but when Jess buys a dog for Leslie, they bring him to Terabithia and in their adventures use the dog as a troll hunter. The troll is used to represent a school bully named Janice Avery, and when Leslie befriends Janice, the troll in Terabithia is tamed.
Later on, Jess is told by his father that the keys to his hardware store are missing and they cost hundreds of dollars to replace. Because his family is struggling on making ends meet, it becomes important to Jess to find them. It turns out that his sister May Belle gave the keys to Leslie, who did not know what they were for and put them in their tree fort in Terabithia. When they next go to Terabithia, Jess and Leslie saw the Dark Master for the first time and race to their tree fort to get the keys. The Dark Master sends his minions after them, and in the fight, Jess and Leslie use pinecones as hand grenades and Jess gains a metallic fist to punch them. Suddenly the Terabithian army arrives to help fight the Dark Master's minions and Jess climbs the tree to get the keys. Leslie uses a beam of light she pretended to capture in the mirror, helping him find the keys. Jess grabs them but falls, and the reformed troll snatches him to safety.
The next day, Leslie is killed in a freak accident while trying to get into Terabitha. The rope she used to get in broke, and she fell into the river, hit her head and drowned. Jess blamed himself for not inviting Leslie on a trip to a museum the day she died. Jess at first is in denial of her death and angry at feeling abandoned. During the emotional climax, Jess returns to the woods, where he lashes out at May Belle for trying to follow him, after thinking he might find Leslie and he is chased down by the Dark Master. Jess eventually stops running, and breaks down, finally crying for his friend; it is at this point that it turns out that the Dark Master he was running away from was actually his father. Jess says he is worried Leslie is in Hell, but his father tells him, he may not know that much about God, but he is sure that he would never send a girl like Leslie to hell. Jess says he thinks he is going to hell because he blames himself, but is told not to think like that for a minute. Jess is told that Leslie gave him something special, and he needs to hold on to their friendship and what she taught him for her sake. With Jess and his father reach an understanding and reconcile their differences, and Jess finally accepting Leslie's death, the Dark Master disappears forever and peace restored to Terabithia and its people, including the reformed troll Janice with Jess Aaron's sister May Belle invited by her brother to join them as their princess.
In addition to the Dark Master representing Jess's problems (mainly his father), the Dark Master's own minions represent other people in Jess and Leslie's lives. There are the Hairy Vultures and Squogres, which represent 2 school bullies Gary Fulcher and Scott Hoager, as well as the troll who represents Janice Avery. These symbolisms can be seen when the troll reforms along with Janice, and during a fight with the Squogres, Jess punches one with a metallic fist, and later punches Scott across the room.
The Dark Master is similar to the Red Knight from the film The Fisher King. Despite not technically existing, they are major antagonistic forces and manifestations of the protagonists' problems (such as grief and depression). Keeping with this, in their respective films, there is an emotional and dramatic scene where they chase the protagonist down.
The Dark Master is also similar to the Pale Man from Pan's Labyrinth. Both are evil figures in the fantasy world of the protagonist('s), and parallels the protagonist's father/stepfather. Incidentally both films have a protagonist dying.
It is common to find the Bridge to Terabithia fanfictions that depicts Terabithia being real and hence the Dark Master. Those fanfictions often involves revealing Leslie was not killed, but abducted by the Dark Master and is imprisoned in Terabithia.
Despite being the main antagonist, he only has 32 seconds of screentime in the film.