Gadflow is a corrupted Winter Fae who seized control of the Winter Court and used his army, the Tuatha Deohn, to wage war on the young races in the name of his to-be-born goddess, Tirnoch. He is the leader of the Tuatha Deohn.
Before this, he was the jester of the Winter Court under High King Mathon. According to lorestones, Gadflow, playing his role as jester, took Mathon's crown and placed it on his head, claiming to be doing so as a jest. He then slew the whole Court of Winter afterwards and then took the place as the high king of the Winter Fae.
It is unknown exactly how Gadflow and Tirnoch came into contact, or if Gadflow was hungry for power before hand or if Tirnoch had twisted his mind. What is known though, is that Gadflow came into contact with Tirnoch, and used her power as his own to wipe out the Winter Court and the Winter Fae heroes who rose to defeat him and stop his mad bid for power.
Gadflow gathered an army and called them the Tuatha Deohn, finding the greatest crafters in the lands of the Winter Fae and having, or forcing if necessary, them to craft items and weapons to be used in his war on the mortal races. As the years went by, Gadflow had lost more and more of his own sanity, spending much of his time in the Amethyn caverns below Bhaile and becoming a twisted and deformed being. His fanatical devotion to Tirnoch has rubbed off on many the Tuatha and the flock to listen to his rants at Chantries that have been set up around Winter Fae territory.
When the Fateless One attempted to assassinate Gadflow and reseal Tirnoch, he was either present or at least aware of his/her death. Shortly afterwards, he became aware of the gnomish Professor Hugues' Well of Souls project and sent troops to infiltrate Dalentarth and destroy the Allestar Tower.
After Mel Senshir and the Fateless One and his/her companion's assistance, pushed through all obstacles to face down Gadflow. Gadflow clashed with Alyn Shir and him/her in the Court of Winter with his avatar. His true self was actually deep below Bhaile in the Amethyn caverns where Tirnoch's mortal form resides.
When the Fateless One comes face to face with Gadflow and Tirnoch, Gadflow seeks her approval for all of his actions. To which Tirnoch responds that he is no longer useful to her and is promptly killed by her.
Taking Vengeance - Main Antagonist
The quotes added here are going to follow a single line of argument. There are always two choices to respond to Gadflow (there is also the possibility to persuade him in one or more of the few conversations you have with him throughout the game), but for not making much trouble I'll let only one.
Be as it may, the player can choose the one response that fits him/her better.
Gadflow has powerful magic that his goddess has given him, some powers include "magic mines" and the ability to appear where he is not.