|“||Necrodeus hates light and wants to steep the world in darkness. As a pirate, I can respect that. But with no light, all my treasure would cease sparkling. And I can't have that. Do everything in your power to stop him. My gold must shine!”|
|— Daroach • Kirby Mass Attack|
Necrodeus' head is a horned white skull with lustrous red eyes. He seemingly has a third eye socket in the middle of his forehead, but no eye is seen within. Necrodeus has two curved horns on the sides of his head and a smaller, straight third horn on its top. Inside his mouth lies a set of sharp fangs, and he has an uncharacteristically wacky-looking clown nose. His body is made of purplish black smoke, which during the beginning cutscene almost resembles Nightmare's cloak. Around his neck is a necklace made completely out of Skully-like skulls. Matching the rest of his cadaverous features, his hands are skeletal and can come together to create a red eye-like marking on his wrists.
He initially wields a black staff with a purple jewel in its top which he uses to decimate Kirby at the beginning of the game, but swallows it in preparation for the final confrontation, requiring Kirby to destroy him in order to retrieve it. As the final battle concludes, his true form is revealed to be a purple Zero-like eye (especially similar to its small red core) that lies within the jaws of his skull.
As Kirby nonchalantly explores the southern reaches of Planet Popstar, a "skull-faced fiend" comes into view and blocks out the sky with a cover of ominous dark clouds. The fiend is instantly identified as Necrodeus, the grievous leader of the Skull Gang, whose goal is to drown the planet's light in an endless sea of darkness.
Using his magic staff, he splits Kirby into 10 weak copies, which he deftly overwhelms until only one remains. Necrodeus disappears, unknowingly leaving the last Kirby to recover using his heroic heart. Kirby sets out on his adventure to claim the staff to regain his former power.
Throughout the game, Necrodeus uses his Skull minions to try and subdue Kirby rather than face him head on. This tactic is unsuccessful, and the Kirbys manage to annihilate the gang, ending with Skullord in Volcano Valley. The Kirbys collect every Rainbow Medal and use them to disperse the cloud surrounding his island, Necro Nebula. With his minions defeated and his hold on Popstar weakened, Necrodeus has no choice but to allow them into his stronghold in Necro Nebula.
Just as the Kirbys think they've claimed the staff, Necrodeus appears and absorbs it, setting the scene for the final battle.
Necrodeus is a multiple-phase boss with a wide array of attacks. Although his attacks are slow and very easy to anticipate, they can all KO a Kirby in one hit. The fight is divided into three parts. The first phase consists of Necrodeus using his skeletal hands to smash the ground below him. As he raises his hands, this gives the Kirbys the chance to move out of the way and avoid the hands when they come down. When Necrodeus raises one of his hands really high and holds it in place for a while, Necrodeus is preparing to slam his fist into the ground. When that happens his fist will be stuck in the ground giving the Kirbys the opportunity to break the orbs on the back of his hands. When one of the orbs on his hands is destroyed, Necrodeus will call Skullions to repair it, so the player must destroy the orb on his other hand before they repair the first hand. When both of his hands have been damaged, Necrodeus will release a ferocious roar, starting the second phase of the battle.
In phase two, Necrodeus will attempt to close his palms around the Kirbys. The Kirbys have to force his hands to the ground as best as they can without getting crushed. When the hands are on the ground, the Kirbys must destroy Necrodeus' hands by pummeling the orange circles that are on them. Necrodeus can use his index fingers to fire sparks of electricity at the Kirbys. When both of his his hands are fully destroyed, Necrodeus will release another roar, destroying the ground and switching off the gravity, causing the final phase of the fight to begin.
With only his head (and to an extent his torso) left, he will begin the final phase of the fight by spitting out metal mines that shoot lasers in different directions. However, these mines can be destroyed by the Kirbys before the lasers are produced. Necrodeus can also discharge large balls of electricity that bounce around the screen; a giant laser can also be shot from his mouth. Necrodeus also can summon Skullions. Like normal Skullys, they attempt to snatch a Kirby from the player's grasp. If a Kirby is grabbed, the Skullion will float over to Necrodeus, who will devour the Kirby and eliminate it from the fight if the other Kirbys are unsuccessful at retrieving the newly defeated Kirby.
At this point of the battle, his weak point is the purple eyeball inside his mouth, which will occasionally reveal itself in the middle of attacks. If the eye takes enough damage, his skeletal helmet will crack and shake rapidly. If the head is subsequently attacked, the player will be required to tap as fast as they can. If the player is quick enough, Necrodeus's cranium will be destroyed and the battle will end.
Once Necrodeus is defeated, his staff will appear, ending the game and allowing the 10 Kirbys and their heart to become one whole being again.
- Necrodeus seems to emit a deep metallic cackle in battle, making him the one of five final bosses to cackle during the fight against him, the other four being Drawcia, Marx, Marx Soul and Queen Sectonia.
- Necrodeus, Dark Mind, Wham Bam Jewel, and Magolor Soul are the only bosses in the series to have exactly three eyes.
- Necrodeus's name translates to "God of the Dead."
- Necrodeus's appearance changes from the cutscenes to the boss battle; he gains hands with a less bony appearance, his horns are a bit more curvacious, and his shadowy cloak is smaller, as well as having a smaller nose and more promient teeth.
- If a Kirby is lost to Necrodeus and the other Kirbys succeed in defeating him, a white Kirby with angel wings will appear in the cutscene after Necrodeus's destruction.
- Necrodeus is the only undead final boss in the series whose living form is never seen or attacked by Kirby.
- Necrodeus and Magolor Soul both have an eye with a red iris in their mouths.
- In a "Behind the Scenes" Miiverse post, director Shinya Kumazaki mentions that the Sun Stones in Kirby: Triple Deluxe contain the power of sunlight, and that a character who "dislikes the sun" may have turned the light into gems in an effort to "rob Floralia of its light." This comment may imply that Necrodeus is the creator of the Sun Stones.
- A slightly distorted version of Necrodeus's laugh is produced by a Grab Hand when he snatches Kirby in Kirby and the Rainbow Curse.