Bloodworms are creatures encountered in the Fallout 4 add-on Nuka-World.
Background
Originally an exclusively aquatic species prior to the nuclear apocalypse, over the last 200 years, bloodworms (genus Glycera) steadily migrated in-land from the coastal waters of the East Coast after being mutated by radiation and have made their homes in the dusty, arid regions of the wasteland, including the low-lying valley west of the city of Boston containing the pre-War Nuka-World amusement park, where one nest led by a queen took over the Dry Rock Gulch park exhibit by 2287.
Despite the total change in habitat from that of its ancestors, the bloodworm has retained some of their original traits, including a burrowing ability, only now instead of burrowing into wet sand and silt, their four hollow jaws have become adept at plowing through the tough and dried soil. This allows them to execute ambush attacks on prey from beneath, and then returning underground to catch their prey unaware again.
Characteristics
Biology
A species of insectoids that have been mutating and evolving since the Great War, bloodworms are now approximately the size of an adult human being and have spines on the outside of their body in addition to their four-pronged maw. These worms react to the presence of intruders in their territory and will charge the intruders by burrowing through the dirt at high speeds. Upon reaching their prey, they will lunge forth from the ground and attempt to bite their target.
Bloodworm larvae will also inhabit the corpses of large prey, such as brahmin and brahmiluffs, and come out of the corpse to attack any new prey that wanders by. These egg-infested corpses can be spotted from a distance if watched closely, as they will twitch and bulge from the movements of the creatures inside.
Gameplay attributes
Bloodworms have two targetable sections, one being their head and the other being their tail. Like mole rats and radscorpions, they can close the distance quite rapidly by burrowing into the ground and bursting forth closer to the Sole Survivor or their companions. However, unlike other creatures that are able to burrow underground, bloodworms have the unique ability to conceal their movement with a dirt cloud they produce while they move on the surface, making them hard to spot when they are about to strike. This does not prevent them from being targeted with V.A.T.S., though.
They are not usually dangerous, as most of the bloodworm species possess very low health, but if found in groups, and allowed to surround the player character, they might be a problem, especially if Action Points are unavailable to help spot them with V.A.T.S. It is possible to prevent them from bursting close to the player character after burrowing underground by standing on materials other than earth, like rock, metal or wood when engaging them. Shooting infested brahmin will leave the bloodworm larvae momentarily stunned before attacking.
Variants
Bloodworm larva

The weakest bloodworm variant.
Name (Form ID) | Statistics | Behavior | Abilities | Items | |||||
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Bloodworm larva xx0138B7 |
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Venomous bloodworm larva

A slightly stronger variant of the bloodworm larva.
Name (Form ID) | Statistics | Behavior | Abilities | Items | ||||
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Venomous bloodworm larva xx0201DA |
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Glowing bloodworm larva

A stronger variant that emits small amounts of radiation.
Name (Form ID) | Statistics | Behavior | Abilities | Items | ||||
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Glowing bloodworm larva xx0201DB |
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Bloodworm

The most common bloodworm variant.
Name (Form ID) | Statistics | Behavior | Abilities | Items | ||||
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Bloodworm xx0201EF |
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Bloodworm queen

A unique variant of which only one is found in the game, located in Mad Mulligan's Minecart Coaster in Dry Rock Gulch. Slightly stronger than the standard variant.
Name (Form ID) | Statistics | Behavior | Abilities | Items | ||||
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Bloodworm queen xx046CF9 |
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Venomous bloodworm

A more powerful variant of bloodworm.
Name (Form ID) | Statistics | Behavior | Abilities | Items | ||||
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Venomous bloodworm xx0201F0 |
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Glowing bloodworm

The strongest bloodworm variant.
Name (Form ID) | Statistics | Behavior | Abilities | Items | ||||
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Glowing bloodworm xx0201F1 |
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Appearances
Bloodworms appear only in the Fallout 4 add-on Nuka-World.
Behind the scenes
The creature's design and behavior resemble that of the Graboid species from the Tremors films. Graboids are massive, subterranean worm-like monsters allegedly existing on Earth since the Devonian era that burrow through dirt to attack targets from below, causing detectable seismic activity, of which is referenced in-game by Ricky's note, with the author claiming that "unusual tremors" are a symptom of the presence of bloodworms.