Gunmar the Black is is the main antagonist of Trollhunters. He is the warlord of the Gumm-Gumm Army and father of Bular. He has been imprisoned in the Darklands for centuries along with his army of Gumm-Gumms by Deya the Deliverer.
Nothing is known about the appearance of Gunmar, but when he is seen through the Killahead Bridge, he appears to have lost his right eye and seems to be a glwoing and bright teal color.
Gunmar was born from the first Heartstone of the Troll race. When the wars between Trolls and humans were unleashed by the domination of the surface, the horrible deaths and bloodshed corrupted the Heartstone, causing it to rot from within, and from inside it Gunmar was born. The first of his many victims was the king of the Quagawumps. He fought with several Trollhunters, however, none were able to defeat him even with the Sword of Daylight.
He fought against Orlagk for the right to be the warlord of the Gumm-Gumms, a weeks-long battle that ended with Orlagk's death and the loss of Gunmar's eye. After the Troll realm and the human realm separated, Gunmar led the Gumm-Gumm Army to take the power of the surface and to devour all of mankind. This unleashed a war between the Gumm-Gumms and the rest of the Trolls that ended when the Trollhunter, Deya the Deliverer, defeated Gunmar, exiling him and all his army to the Darklands.
Centuries later, his son Bular teamed up with the Janus Organization to piece together the pieces of the Killahead Bridge under the guise of the Arcadia Museum, headed by the chief curator and fellow changeling Nomura, Otto Scaarbach and Walt Strickler.
Upon the Bridge's completion, Gunmar was able to speak to his son and his subordinates via projection. Bular then informed Gunmar that the current Trollhunter was a human named Jim Lake Jr. and that Strickler had forbidden him from taking the Amulet of Daylight, the only way to open the Bridge, from him. Strickler, in turn, argued that killing a human child would cause too much scrutiny, but their bickering only annoyed Gunma. The Gumm-Gumm warlord acknowledged that Strickler was right to spare the Trollhunter so as to keep their plan secret before declaring that Bular shall be in charge. He then sent everyone away so he could speak with Bular privately, suspecting his son had misgivings about Strickler. Bular told Gunmar that in his human guise, Strickler was Jim's teacher and most likely felt affection for him. So Gunmar instructed Bular to subtly reveal Strickler's identity to Jim so the two would be forced into a confrontation in which Strickler would kill the Trollhunter and take the amulet.
Strickler did indeed steal the amulet, but swapped it for a fake to fool the Trollhunter. However, as everyone discovered, only the Trollhunter could use the amulet to open the bridge. Despite being engaged in battle by Jim and his friends, Strickler is able to force him into opening the Bridge, allowing Gunmar to call out to Bular. The bridge was eventually destroyed by Draal, who removed the amulet before Gunmar could be freed from the Darklands.Upon learning of the death of his son, Gunmar sent Jim a message through Gnome Chompsky, who had been sent into the Darklands through a Fetch some time prior, threatening to murder everyone Jim had ever loved for killing Bular. In the threat, translated by Blinky from Gnome Chompsky, Gunmar promised an 'ocean of blood' for Jim and his loved ones, that he would 'make a throne from their bones'.