Arachnitor is a spider themed Nighlok. He served Master Xandred at first but then betrayed him and decided to usurp him and become a ruler of Nighloks himself. Arachnitor is notorious nighlok and is greatly feared by all nighloks, including Octoroo. He is also one of the strongest monsters in the entire Power Rangers franchise. Arachnitor serves as the main antagonist of the episodes "The Tengen Gate" and "Super Samurai".
He was voiced by Simon McKinney, who also previously voiced Splitface during the Samurai season.
Arachnitor wanted to get rid of Master Xandred in his desire for power. His plan included using the same sealing symbol that was used on Master Xandred in the past generation, but needed Octoroo's help to get it. When Octoroo poisoned the team's drinks while they were at Tengen Temple, Jayden took a sip and became sick. While Arachnitor took on the Samurai Rangers, Octoroo demanded the sealing symbol in exchange for the antidote that would cure Jayden. With Antonio not knowing about Deker's true identity, he wanted him to try out his cooking, but when he heard about the Nighlok attack, he was too late as his friends were gravely injured and learned the truth about Deker as he took Jayden to a faraway place. As punishment for defying Master Xandred, Arachnitor was taken back to the Netherworld and was given Master Xandred's newfound power and by the next episode, he becomes a mindless Nighlok.
In "Boxed In", Jayden explains after defeating the Giant Moogers that it was true that Arachnitor had already escaped to the Netherworld. Arachnitor returns in Super Samurai leaving footprints from a puddle of Sanzu River Water making it known that a Nighlok is staying in our world without drying out by using that puddle. He is first seen terrorizing a suburban area by the rangers and puts up a good fight until he escapes again from the rangers.
The rangers follow him to what it seems to be an abandoned worksite with an area covered in puddles of Sanzu River Water with some back-up of Moogers. He is first defeated when Antonio gives Jayden the Black Box having Jayden going into Super Samurai Mode performing "Super Blazing Strike". He is defeated again when the Claw Armor Megazord is formed for the first time and it uses the Samurai Battle Cannon to finally put an end to Arachnitor once and for all.
- Arachinitor is one of the few monsters who is not destroyed in the episode he is introduced in. In fact, he does not actually get destroyed until the second half of the next season of Samurai. He is also the first Mutated Nighlok.