Arachnitor is a Ushi-oni-like Nighlok who wants to get rid of Master Xandred in his desire for power. His plan includes using the same sealing symbol that is used on Master Xandred in the past generation, but he needs Octoroo's help to get it. When Octoroo poisons the team's drinks while they are at Tengen Temple, Jayden takes a sip and becomes sick. While Arachnitor takes on the Samurai Rangers, Octoroo demands the sealing symbol in exchange for the antidote that would cure Jayden. Antonio, who has been distracted by cooking for the man he does not know is Deker, arrives to find everyone gravely injured, and he helps them take Jayden to a safe place. As punishment for defying Master Xandred, Arachnitor is dragged back to the Netherworld and is mutating thanks to Master Xandred's newfound power.
In "Boxed In", Master Xandred's power turns Arachnitor into a mindless Nighlok and Octoroo is ordered to send Arachnitor back to Earth once he is finished mutating. When that is done, Arachnitor is sent to Earth and attacked the city. While Antonio works on the Black Box by adding the powers of each Power Disk to it, the other four Rangers fight Arachnitor. Although Arachnitor overpowers the Rangers, Jayden and Antonio arrive in time to gives the Rangers their Power Disks back. The Rangers are able to knock Arachnitor down with the Quintuple Slash. Combined with the Rangers' Quintuple Slash, Antonio uses a spin attack to knock down Arachnitor as giant Moogers attack the city causing the Rangers to form the Megazords. While the Rangers fight the Giant Moogers, Arachnitor escapes. Once the Giant Moogers are destroyed, it was stated by the Rangers that Arachnitor is half-way back to the Netherworld by now.
In "Super Samurai," Arachnitor resurfaces and is causing despair near a forest. Mike uses his knowledge of the forest to find Arachnitor with the other Rangers. Emily points out he's the one that "got away". He is able to knock down the Rangers before he flies away to a seemingly-abandoned work site where there are puddles of water from the Sanzu River. Using the power of the Black Box, Arachnitor was defeated by Red Ranger/Super Samurai Mode. Arachnitor then grows as Octoroo summons some Giant Moogers and two Spitfangs to help Arachnitor. The Rangers form the Samurai Megazord to fight Arachnitor, the Moogers, and the Spitfangs. Arachnitor is finally destroyed by the Claw Armor Megazord and the Samurai Battle Cannon (which was used against Arachnitor and his pair of Spitfangs). The Tengen Gate