As a character, Fabian Cortez has exactly two noteable deeds in the comics, and one is entirely dependant on the other.
The first, obviously, is his introductory arc, in the three opening issues of X-Men: he pledges fealty to Magneto, and uses his (then) ill-defined powers to help Magneto fight the heroes; the end of the arc reveals it was all a scam for personal glory, with Cortez's abilities not doing anything to actually heal Magneto, just boosting the magnetism so the leader felt less injured; Cortez then withdrew his boosting, effectively pulling the plug on Magneto's life support and leaving him for dead. Of course, Magneto was too important a character to die, so he survived the betrayal (and Asteroid M's ensuing planetfall), which lead directly to Fabian's second big moment.
Pissed that he'd failed to kill Magneto, Cortez looked for another way to get at the man. He settled on exploiting Magneto's blood ties by kidnapping Luna, the daughter of Crystal and Quicksilver, and therefore Magneto's granddaughter. This led to a 1993 crossover in honor of the 30th anniversaries of both the X-Men and the Avengers, the first time mutant problems had really had any impact on the rest of the Marvel world since "Inferno," and the first time the two teams had met since 1987's X-Men vs. the Avengers.
Anyway, with Fabian holed up on Genosha, both groups separately come looking for him, and it eventually gave us one of the funniest bits in any X-Men or Avengers story for the entire decade:
Fabian Cortez's role in the crossover tends to be forgotten, because he got exploded in the middle of it and the second half was really a showcase for the newly introduced Exodus, but it definitely started because of him.
