Dagda Mor is the main villain of The Elfstones of Shannara, the second book of the Shannara series by Terry Brooks and the main antagonist of the first season of the Shannara Chronicles TV show.
He's the ruler of an army of demons, which were banished in another dimension called the Forbidding by the elves eons before the events of the book. Then the ancient elves used most of their magical power to create a silver tree with the fusion between a woman and a fire seed.
The silver tree works to keep the demons in the other dimension, and in the beginning of the events of the book, it started to die, allowing the Dagda Mor and his right hand man, the Reaper, to return on Earth.
Dagda Mor sends the Reaper to kill Will Ohmsford and Amberle, the ones chosen to recreate the silver tree.
Towards the end of the story, Will manages to defeat the Reaper and Amberle sacrifices herself to become the new silver tree, sending Dagda Mor and the demon army back in the other dimension. (On the show, Allanon chops his head off before the demons are banished)