Necrodeus serves as the main antagonist in Mass Attack. He appears as a "skull-faced fiend" that comes into view, blocking the sky with dark-filled clouds. He uses his staff to split every character of the Locked Room gang into 10, and defeats all but unknowingly one of each character to recover using their heroic hearts. Throughout the adventure, he sends his Skull minions and subdue the characters. It wasn't until the near end of it where he appears again, this time holding a rotten Floro Sprout and preventing the characters to hold the magic staff. After the final battle in that room, he is defeated. He nearly fades into oblivion but it wasn't until he will appear again in a future room.
Not much is known deeply about Necrodeus' personality, even in the game. However, he is known to hate light and wishes to perish it as his goal. His Skull minions follow his orders, no matter what, showing that his leadership is nothing to be underestimated.
Necrodeus is a "head and hands" final boss. In phase one, he uses his hands to attack, crushing characters to the ground. However, he sometimes gets those hands stuck to the ground as well and it's the chance to attack them. After that, he will try to crush the characters again but with the palms. The middle is its weak spot. Shortly, he will launch 3 energy balls on his index fingers.
In phase two, Necrodeus uses his head. He spits mines that can be destroyed but can fire lasers in a crossed shape. He fires large energy balls that bounce and last longer than the normal ones. He can fire a large orange laser but will reveal his head's weak spot, which is its eyeball.
- Necrodeus and Daroach were originally going to be free-use, but Fairy27 ultimately decides to own them both.
- While Daroach will just leave after the plot of Mass Attack (but potentially come back in Season 3), just like on its game counterpart, Necrodeus will not be limited on this room for reasons.
- Necrodeus' name translates to "God of the Dead".
- The director's (Shinya Kumazaki) comment in a Miiverse post about Sun Stones in Kirby Triple Deluxe, which refers to an unknown character that "hates the sun" may have turned the light into gems to "rob Floralia of its light", may refer to a guess where Necrodeus is the "unknown character". One action of this is to be revealed.