I traced Black Scorpion's history back to a mercenary in Nicaragua named Ernesto Rodriguez.
Rodriguez hired out for various little wars until he was chased north by the government, at which point he ended up in Mexico.
At that point he began hiring himself out for various jobs across the border into the US. It was on one of these missions that his life changed forever.
Rodriguez was leading a recon mission to investigate a hidden laboratory run by Leonard Bowen. Bowen was known throughout scientific circles for being brilliant but unsafe.
The mission did not go well at first. Rodriguez and his men were spotted by a sentry. It quickly turned into a bloodbath. Details become sketchy at that point but it seems that when he finally got to the lab itself, Rodriguez discovered Bowen working on some kind of armored suit.
He apparently tortured and then killed the scientist. Having ripped most of the secrets of the suit from Bowen, Rodriguez became Black Scorpion.
Rodriguez used his newly found power to advance into the super-powered mercenary ranks. His only clear goal seemed to be finding technology to improve his armor. He was hired for an Arachnos mission and realized immediately that having access to their labs and scientists was exactly what he needed.
It wasn't long after that Back Scorpion encountered another mercenary who was equally fascinated with "upgrades". This woman called herself Silver Mantis.
Silver Mantis was born Tamura Shirai. She began modifying her body in her early teens. It began with tattooing and piercing but she quickly discovered a new form of modification; Metal fusion.
Tamura found an "artist" who would fuse smooth, sleek but incredibly strong metal to her body. The cost was astronomical and so Tamura began to steal to feed her habit. Her career started with petty crime, but she soon found that nothing was more important to her than being able to pay off the "specialist" she had found, not even human life.
Her first kill was surprisingly well executed and soon the entire mercenary community realized that she had not only a taste for it, but a talent as well.
All details I uncovered on the first meeting between Black Scorpion and Silver Mantis indicate they were a perfect match from the beginning. He is the ultimate expression of her obsession and she makes him feel like the most powerful being on the earth.
I suppose it is not a surprise that Silver Mantis realized an opportunity when she saw it, and rather than fighting with Arachnos over the ill-gotten loot, formed an alliance that has lasted to this day.
Currently Black Scorpion spends a great deal of his time monitoring the scientists of Arachnos. He uses the ever present threat of violence to create the best technology. Since his personal goals serve Recluse's plan, I believe that is the reason the Lord of the Rogue Isles gives him free rein.
Black Scorpion and Silver Mantis present a great physical threat. He is a Brute, strong and extremely heavily armored. His tail can launch energy attacks as well as vicious whip strikes. Ernesto Rodriguez embodies the concept of raw power through technology.
Whatever it takes, whatever piece of his body or soul needs to be spliced, altered, or removed to grant him more power, Ernesto seems willing to do it. His unbridled hunger for power and low intelligence make him the perfect servant for Lord Recluse.
Silver Mantis is a Stalker, and has somehow gained control over the metal spikes that are screwed into her bones. I believe it is a mutant ability that allows her to fire the spikes from her body in bursts. She also derives great pleasure in inflicting pain and torturing her victims before killing them.
Her unbending loyalty to Black Scorpion makes her a deadly tool for Arachnos to exploit.
Although some of the other arch-villains in Arachnos present a more strategic threat, Black Scorpion is rivaled only by Captain Mako for sheer brutality. Perhaps that is the source of the conflict between the two villains. It is possible that their dislike for each other can be used to help defeat them.