Plasmius is a Shadow Boo, which is a Boo from the Shadow Realm. He is immortal, due to him being dead.
Plasmius originally was a Shadow Being, and great swordsman. He decided to join an unknown army, and the plan was to build a kingdom of Darkness. However, Princess Peach's ancestors and Mario and Luigi's ancestors(As well as Parax himself) were there first, and a war began. It lasted for thousands of years, and Plasmius was one of the 10 survivors of his army. He was captured and put to death in the castle after it was built. Now he tends to haunt the castle, swearing revenge on those who put him to death. He gained several abilities after death, but he is still not too fond. However, he was able to join the Boo Army, and after showing so much loyalty, was brought to Lord, the level right below King.
A rivalry with the Mario Brothers Edit
Mario was hearing weird noises in the castle, and was about to tell Parax to check it out, but Parax, reading his mind, told him he was very busy studying an artifact from his last trip to a jungle out on an island (the natives thought he was a sort of god, because he gave them a chocolate bar and Pasta, as well as kill a monster that was constantly attacking them). Mario asked Luigi, who refused, because he was reading a book to help Parax study the artifact. Mario sighed, and went upstairs to the top of the highest tower, and he heard loud banging noises. He saw a portal, and entered. He was in a secret room of the castle, and saw a painting with Plasmius in it. He was banging on the painting, yelling angrily. Mario walked closer to get a better look, and Plasmius told him that he had been sealed away by monsters, and that he could only be freed if he got his crown returned to him. He said it was buried next to the cannon outside the Castle. Mario had a bad feeling about it, but went anyways. He dug up the crown, and placed it in front of Plasmius' painting. The crown was sucked into the painting, and Plasmius pushed his way out of the painting. He turned to Mario, and roared. Mario screamed for help, but was knocked against a wall. Before Plasmius could have a quarter of the revenge he wanted, Parax appeared just in time and fired a blast at the ghost. Plasmius merely laughed, and disappeared. Parax glared at Mario and it was easy to tell he was angry. "What were you thinking? If you see a black ghost who looks like....bleh...you DON'T free it!"
After years of fights with the Mario Brothers, Plasmius kidnapped Peach in an attempt to perform a ritual, so he could be revived. The Mario Bros. arrived, but were defeated easily. Parax, who left the Shadow Army, mixed with Mario so their powers were combined. Mario, taking the form of Omega Mario, blasted Plasmius out into space. He landed on Rosalina's Comet Observatory, and this started a rivalry between them. Plasmius, who was about to use Rosalina for his ritual, was surrounded by Lumas(which were freed by the Mario Bros.). They then carried him to the Mario Brothers(and Parax, who left Mario's body), and they blasted him out into space once again. He returned to the Shroom planet to try and start a war between the Mushroom Kingdom and the Undead Army. This failed, and he was forced into hiding.
Plasmius can turn invisible completely, and can pass through walls. He can go into paintings and hide in them. He can use plasma attacks, which he was born with, earning his name.