- "I call this one the Nether Saber, and some day, it'll shine as brightly as the Sun."
- —Merlin during construction of the Nether Saber
The Nether Saber is an ancient beam sword, created by Merlin to defeat Nox Decious. After their battle, Merlin deemed the Saber too powerful so he split its power into three separate objects: The Nether Emblems.
When Decious tried to control the world again, Mario, Luigi, and their friends gathered the emblems to reconstruct the saber because they learned it was the only weapon that could stop him. Wario was the one who merged the emblems together and killed Decious. Ever since, the Nether Saber has stayed with him. The Nether Saber is so powerful that its user is guaranteed to win every battle using it, even though it can be knocked out of the user's hands and temporarily stolen.
5,000 years ago, Merlin created the Nether Saber as one of his first beam swords. He first showed it to Nox Decious during one of their conversations, with Merlin claiming it would glow as bright as the sun one day. However, Decious didn't believe that the technology would work.
4,000 years afterward, Merlin accused Decious of being the cause of Uther, one of Merlin's presumed Chosen Ones, to betray him. The two got into a fight, with Merlin using the Nether Saber while Nox was charging a ball of shadow. Merlin slashed Decious across the face with the saber, resulting in it being horrendously scarred forever. After his battle with Decious, Merlin split the Saber into the three Nether Emblems, deciding that it was too powerful to be used again.
After it is constructed, Mario takes it to face Nox Decious, but to no avail as it is knocked from his hands. After a fight between Mario and Decious, Mario is being chocked by Decious, but is saved at the last second by Wario, who uses the Nether Saber to kill Decious.
Wario tries to kill Mario with the saber, but it turns off at the last second. After Mario flees, Wario hears a mysterious voice which tells him to see Nilrem, who can help him recharge the saber. It is revealed that the only reason that the saber was able to kill Decious was because it was made from objects that he loved in life. None of the Nether Emblems have any special meaning to Wario, therefore, as long as he had the Truth with him, the saber can not kill him, making him immortal. However, since he knows the Truth could be stolen from him, Wario makes a plan to get the Nether Saber recharged by deliberately dropping it and luring Ash into picking it up. Solid Snake is instructed by Master Miller, who is actually Wario in disguise, that the Nether Saber can be deactivated by placing it in the hole in the middle of Merlin's table, the place it was created. They don't know that this actually recharges it completely before being ambushed by Wario, who retrieves the saber and uses it in the final battle with Mario, Luigi, and Waluigi.
After the said battle, the now yellow Nether Saber is kept in the possession of Wario, who teams up with Mario, Luigi and his friend Waluigi to defeat the Darkness.
Wario uses the Nether Saber to battle the Darkness in the battlefield in "Act II Part 3." In the end, the Darkness almost kills Wario with the Black Widow Blade, but thanks to the Truth Stone, he lives. The Darkness thinks that Wario died, but Wario, who is still alive, throws the Nether Saber through the Darkness' back and he supposedly dies.
Later, Waluigi tells Wario in "Act II Part 6" that the Darkness is still alive because he didn't deliver the Counter Curse, which turns the Darkness human. So, Wario confronts the Darkness one last time in the Battlefield. Wario pins him down, says the Counter Curse, and summons the Nether Saber with the force and delivers the killing blow to the Darkness. It is unknown what happens to the Nether Saber after this as Wario throws it aside and appears to walk off without it.
The Nether Saber's only appearance in this season is briefly in Mario's dream in the season opening, and isn't seen since, but it is mentioned by Wario once in the season finale. Wario explains to Mona that the Nether Saber's power is connected to Merlin and that it would not work without Merlin being alive. Wario says he tests the saber everyday and it works perfectly.
The Nether Saber makes its final appearance in this season, where Wario uses it against Nox Decious. It is revealed that Merlin is dead, but his spirit lives on, which allows the Nether Saber to keep working. When Merlin transfers his power and spirit to Mario, it can be assumed that the Nether Saber has been deactivated or the Nether Saber is now connected to Mario.
By "Bloody Confrontations," the Nether Saber had lost the green glow it previously had. The core's color represented Wario's inner struggles as he isn't truly evil. When Wario realized he was the second chosen one, the saber reverted back to its usual appearance.
During Wario's first battle with the Darkness, when the latter briefly took the Nether Saber from the former, the core instantly changed from white to black, due the the Darkness being pure evil. The core remained this way until Wario threw the saber through the Darkness' back, after which it changed back to white.
- When the saber appeared in Oops! We Broke the Table Part II, it glowed a greenish color when Merlin used it. In "The Super Mario Brothers," the blade glowed yellow when he used it. The change from green to yellow was presumably to give Wario a blade that matched his appearence, and its appearance in "The Super Mario Brothers" was to not create a continuity error.
- If the Darkness had stabbed Wario with the Nether Saber in "Act II Part 3" of the film, Wario probably would have died because the Truth Stone can't protect against it. That was the case with Nox Decious, but in the beginning of Something Crazy This Way Comes, Merlin explains how the Nether Saber was only able to kill him because it was made from 'things he loved in life,' and as they have no special meaning to Wario, the Nether Saber probably can't kill him if he has the Truth Stone.