Quite a pair are these two. One will rob you blind, while the other kicks your ass. I'm not quite clear on how these two met (my back issue collection on X-Men history is very limited), but I do know that there has never been a closer relationship between villians like this.
Black Tom Cassidy is the first cousin of former X-Man Sean Cassidy, AKA "Banshee". Tom and Sean were both in love with an Irsh maiden named Meave Rourke, but shortly after marrying Sean...Meave gave birth to X-Force member Theresa Rourke-Cassidy (Syrin), and sadly Meave died while in labor. After returning home to Ireland (after being on assignment for Interpol) and learning of his wife's death, Sean had left the country in a fit of rage. Leaving his daughter unknowingly behind to join the X-Men. Tom assumed custody of baby Terry, he took great care of her and always provided for her the best he could, even through illeagal means. After becoming so immerssed in the Black Market (hence the name), Tom enroled Terry in a boarding school to protect her. After several years dealing in the business of crime, he met his good freind and partener The Juggernaut.
Cain Marko's childhood wasn't quite as privlaged as his half-brother Charles Xaxier (Professor X). But during his service with the Army during the Korean War, Cain stummbled upon the temple of Cytorak. There he found its Crimson Gem which transformed him into the hulking monster known as the Juggernaut. Since that time, he's been a constant thorn in the side of all the X-teams. After shakey partenerships with Magneto, Mr. Sinister, and The Brotherhood of Evil Mutants, Juggernaut came across the path of Black Tom, and their mischief together has always been entertaining.
Thier start of troubles with X-Force began when Tom agreed to take hostage many of America's wealthiest business men for the sake of saving the Stankos corporation, in exchange for the rescue of his freind Juggernaut from the void between realities. Both plans were sucsessful at first, but when X-Force came calling for the release of the hostages, things got out of hand, and the two were defeated by the combined works of X-Force and Spider-Man (X-Force #'s 1-4, and Spider-Man #16). The duo were spoted once again together as they were hired by Scarmore Inc. to kill Venom. But as usual they learned exactly how bad the situation was after it was too late to do anything about it (Venom "the madness" #1-3).
Soon after that, The Juggernaut was swept away to the Ultraverse where he alongside the "Exiles" with fellow Marvel characters Reaper (from the MLF), and Sienna Blaze (of the Upstarts). But while Juggles was fighting alongside other friends in the Ulraverse, Tom didn't fare out so well. It was this time (i'm estimating) that Tom caught the "Legacy Virus". Of course he still had a job in the Black Market, so it wasn't until he heard the convincing from his adopted daughter Theresa to turn himself over to the authorities, that he could fianlly recieve the help needed for the disease (X-Force #31). He hasn't been seen alive since then, but his death has not been anounced as well. As for Juggles? Well, he eventually came back to the Marvel Universe, but only to get killed at the hands of Onslaught (X-Men #54).
