The King Worm  (also known as Beam Beast in Metroid: Zero Mission: The Official Nintendo Player's Guide, X-56V, Deorem and Diorumu ), is the first boss Samus Aran may face in Metroid: Zero Mission on planet Zebes (as well as the entire Metroid series).
Once Samus obtains her first Missiles and heads back to the door, she will encounter this creature. It will entrap Samus by using its elongated body to block all exits and force her to fight it. The King Worm attacks by rushing its head towards its target, damaging her with its horns and spikes. The worm's second method of attack is to shake the segment of its body that is closest to Samus; several thorns which cover its body will start falling off and rain down on her.
All Samus has to do is fire a Missile at the creature when it opens its eye. Once Samus defeats it, she will obtain the Charge Beam. If the battle goes on for too long, however, the King Worm will grow tired of fighting and leave. This will happen if Samus does not successfully hit its eye three times.
If Samus does not kill the King Worm in the first encounter, she will encounter it at other times as she explores Brinstar. Samus will later encounter the Mua in Kraid's Lair, so it is possible that the King Worm will not be the first boss she defeats. Because this boss can be skipped, it is even possible to fight it after defeating Mother Brain. It will require one Super Missile to kill.
The creature also appears in the Metroid Manga, but is defeated in a very different way. While fighting it, Samus notes that Zebes's entire ecosystem appears as if it "is trying to protect Mother Brain."
On the official Japanese website for Metroid: Zero Mission, this creature is given the title Underground Demon.
- It is neither revealed nor explained how and why the King Worm possessed the Charge Beam. It is possible that it unintentionally obtained the upgrade while digging through the tunnels of Brinstar.
- Despite the King Worm having the Charge Beam, it never uses the item's abilities, contrary to other beings Samus encounters in her later missions who use their respective Chozo items in some shape or form. A logical theory would be that the King Worm has no beam weapon to begin with, thus making the Charge Beam item useless (one can easily imagine the Bomb item unusable without the Morph Ball). However, the boss' charge attack, in which it attempts to ram itself against Samus, can be a reference to the charge aspect of the item.
- The Rhedogian in Metroid: Other M bears similarities to the King Worm in terms of fighting.
- Although this fact is difficult to detect, the King Worm keeps its eye open during its final dive into the floor while retreating from battle. It is thus possible to kill it in the process of retreating.
- It is also possible to get an extra hit on the King Worm the instant the battle starts, as it opens its eye for a second, making it vulnerable to a missile.