The Skullcrwalers are the main antagonists of Kong: Skull Island. They are monsters which live within Skull Island, and are lead by Skull Devil, alpha of their kind.
Kong: Skull Island
After the Sky Devils Team incident with Kong, Conrad and the survivors are in an old structure which is under the control of an ancient community. There they meet Marlow who tells them that it is not with whom one should fear but a few creatures called Skullcrawlers, monsters who responsible for the death of his partner and are looking to feed to square their neverending hunger. This explained why Kong attacked Conrad and co. because the giant ape aware that their actions will inevitably releasing surviving Skullcrawlers that he drove away from the island's surface, as his parents have gave their lives to stop the monster to destroy all lives on the island.
Jack Chapman, who was one of Sky Devils' commandates, is killed by one of them while he was distracted.
When Conrad and Skydevils arrive, they encounter one of the Skullcrawlers who murder Randa and several members of Sky Devils. During the battle, it is revealed that this Skullcrawler is the one who killed Chapman, and two members of the survivors are in open area that made them easily spotted by predators of the island. Conrad runs to save his friends in an attempt to kill them, San with a lighter manages to kill it.
When Marlow explains the history of the island, he mentions how Kong battles with the skullcrawlers, leading into a flashback showing a short battle between Kong and multiple sub-adult skullcrawlers, these ones being fairly large, almost as big as Kong himself. Kong eventually kills the group of beasts, as Marlow reveals that Kong is believed to only be capable of handling these sub adults.
The existence of larger Skullcrawlers eventually confirmed when a gigantic 95-foot Skullcrawler, which Hank Marlow identified as the "Big One," emerged from the water Packard and his men ambushed Kong in the swamp. The beast attacked the wounded Kong, who had just stomped Packard under his fist, but the humans were able to escape. By sunrise, the humans were attempting to rendezvous with Houston Brooks and Lin San, who were commanding the boat the Grey Fox. However, they were set upon by the giant Skullcrawler. One soldier, Earl Cole, attempted to sacrifice himself by detonating two grenades in front of the Skullcrawler, but the beast simply smacked him away with his tail. Kong soon arrived and smashed a boulder into the Skullcrawler's face, buying time for the humans to escape.
The creature then turned his focus to Kong, pouncing on him and gripping his throat with his jaws. Kong and the Skull Devil continued to brutally wrestle, pummel and bite each other until Skull Devil forced Kong into a shipwreck with a blow from his tail, where the ape became entangled in the chains. With Kong subdued in the wreck, he returned his focus to the humans. As it approached the crew once more, the Skull Devil's right eye was destroyed by a flare that Weaver fired, and this allowed Kong to use and swing the propellor of a wrecked ship at his adversary like a medieval flail, which caught the beast in the shoulder and allowed Kong to resume his fight with it.
During the battle, Skull Devil was also fired upon by Conrad and his companions, allowing Kong to gain an advantage in the fight before finally uppercutting him with the ship's propeller, slicing Skull Devil's throat wide open and seemingly killing him. As Kong saved Weaver, who had been knocked into the water earlier in the fight, Skull Devil pounced at Kong once more. After coiling its tail around Kong, it wrapped its tongue around the ape's arm, pulling it down its gullet in an attempt to devour Weaver. After a brief struggle, Kong tore the creature's internal organs out of its mouth, killing him outright and avenging his parents' death. With that, the threat of the Skullcrawlers presumably ends for good.