Volcancer is a Mirror Monster that is contracted to Kamen Rider Scissors: Masashi Sudo in Kamen Rider Ryuki.
Volcancer was the first example of the consequences that a Rider will face if they breach their contract with a Mirror Monster.
Volcancer appears as Scissor's Contract monster and resembles a humanoid crab. His hands are giant pincers which are occassionally used by Scissors for attacks.
Kamen Rider Ryuki
Volcancer first appeared to be attacking a woman. Shinji, who mistakenly took it for an uncontracted monster fought it before running into his contractor, Kamen Rider Scissors.
After that, when Masashi's Deck was destroyed by Ren in Mirror World during a battle, Volcancer attacked, ate and killed Masashi as the destruction of his deck had also resulted in a breach of his contract with the monster. Volcancer, now uncontracted to a rider, attacked Ren and was initially winning until Shinji appeared. Volcancer was eventually destroyed by Ryuki's Dragon Rider Kick and its essence absorbed by Darkwing when Ryuki told Dragredder not to absorb it.
Kamen Rider Decade
Scissors tricked Verde after the latter was gaining the upper hand, saying that he lost. After Verde let his guard down, Scissors summoned Volcancer to attack Verde, removing from him the trial at the same time.
OOO, Den-O, All Riders: Let's Go Kamen Riders
To be added