Fictional character biography
Onyxx was passed from various foster homes and orphanages, unable to find acceptance because of his crude behavior and chronic bedwetting. However, the latter was cured when young Sidney mutated into the huge rock-like form he now possesses, much to his delight.
Onyxx became a student at the Xavier Institute, assigned to Gambit's training squad, the Chevaliers. Onyxx had a romantic interest in a new student at the Institute by the name of Foxx. It was later revealed that Foxx was actually a long time adversary of the X-Men, Mystique. She infiltrated the school to destabilize the relationship between Gambit and her foster daughter Rogue. After having revealed herself to the X-Men, Onyxx attacked Mystique for not returning his affection. Mystique escaped and, with the help of an accomplice, injured Onyxx.
Onyxx's fellow student Bling was also attracted to Foxx, and their shared infatuation may have been the basis of a friendship, since the two are often seen together. Bling once referred to Onyxx as "Randy," which could be interpreted as a playful nickname ("randy" being a synonym for "horny") but was more likely a writer's error.
Sidney retained his powers post M-Day and continued residing at the school. Onyxx was critically injured by Reverend Stryker during his attack on the institute; barely surviving. Onyxx was later captured alongside his fellow students in Limbo. He is usually shown in the company of Bling. He lived at the Xavier Institute in San Francisco.
Onyxx, along with fellow students Mercury and Loa, was tasked with keeping peace at the riots on Telegraph Hill. This is after the mutant-hate group "Humanity Now!" marched from Sacramento to San Francisco to promote "Proposition X", displeasing many citizens of San Francisco, mutant and non-mutant alike. After the riots subsided on Telegraph Hill, Cyclops came and picked them up, taking them to City Hall. He is also seen in the company of Ariel when she arrives to pick up an injured Trance.
Necrosha and Death
Onyxx is seen amongst those under attack by Selene's new Inner Circle of death mutants on Utopia. Onyxx tries to sneak attack Wither while he is distracted by Dust, but Wither catches him and disintegrates him into rubble apparently killing him. He and the mutant Diamond Lil were casualties in Selene's attempt to gain a mystically charged knife.
Powers and abilities
Onyxx has a craggy granite body which gives him superhuman strength and greater resistance to physical injury. He has also been seen to have a second pair of eyes, but it is unknown if these serve any special purpose.