Gunmar the Black, Skullcrusher, Underlord 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.
He lost his right eye (now the Eye Stone) in battle with Orlagk the first gumm-gumm king. He has two large horns on his head which span out big. His skins glows in a bright teal colored design. He has yellow teeth and a complete lack of hair.
Gunmar was born from the corruption of 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. Only his son, Bular, managed to escape inprisonment in their home, The Darklands. Bular then started scheming for centuries to free his father and eventually allying with Stricklander and The Janus Order to accomplish the goal.
Centuries later, Bular, Strickler, Otto Scaarbach, Nomura, and the Janus Order completed the Killahead Bridge.Upon the Bridge's completion, Gunmar was able to speak to his son known as Bular 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 Gunmar. 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'.
Jim then decides to go to the Darklands all alone, so nobody else gets hurts.
Gunmar is alerted of Jim's presence in the Darklands by Dictatious Maximus Galadrigal. He orders for Jim's capture for he's the only one who can open the Killahead Bridge. Dictatious sends goblins in hot pursuit of Jim, who searches the Darklands for Enrique Nuñez. Jim finds Enrique in the Nursery, escapes the pursuing goblins and goes back to the Killahead Bridge. He activates the portal but seeing this, The Troll Tribunal decides to destroy the bridge, thinking it's Gunmar escaping the Darklands. When the portal is closing, Jim manages to hand Enrique over to Claire, but by then, the portal closes. Immediately afterwards, Jim is taken prisoner by Dictatious and two Gumm-Gumms.Following Jim's capture and subsequent imprisonment, Gunmar requests an audience with the captive Trollhunter. Taken from the dungeon by the guards, Jim is brought before Gunmar in his throne room. There, Jim breaks out of his chains with his sword,attracts the attention of Gunmar's guards and takes them down easily. Disappointed at his guards being overcame by a child, Gunmar drains the soul of one guard, forcing him to fall into a pit, supposedly to his death. Showing interest in Jim, Gunmar attacks, seeking revenge for Bular's death at first, until Dictatious suggests to spare Jim upon seeing a candy sent by Toby and dropped by Jim in the scuffle.
As Jim is dragged back to the dungeon, Gunmar questions Dictatious about his request. Gunmar is shown the dropped candy and realizes the Kilahead Bridge still exists; therefore he could use Jim and the bridge to escape the Darklands. When Jim's friends enter the Darklands to save Jim and bring Nemura with them, Gunmar uses the distraction to go through the open killahead bridge with the now blind Dictatious and two guards, which had been left with unguarded when Draal went through to save Jim and the others.
Gunmar brandishes the Decimaar Blade, a sword cursed with dark magic that can be summoned to his hands and also allows Gunmar to control the minds of his enemies, or turn them into Gumm-Gumms.