In 1991, Nicieza joined with artist Rob Liefeld in co-plotting and writing the final three issues of the "New Mutants". In those issues Liefeld and Nicieza created the characters Deadpool and Shatterstar as well as the super team, X-Force. Liefeld and Nicieza then produced an ongoing "X-Force" title. Nicieza initially worked on the title as scripter; after the departure of Liefeld in No. 12 he became its full writer, which he remained until 1995. By the end of 1992, Nicieza became regular scripter for "X-Men" vol. 2, beginning with No. 12 (Sept. 1992), working primarily with penciler Andy Kubert throughout his run. For the next three years, Nicieza was among the writers and editors of one of Marvel's most popular superhero franchises during a time of such popular, multi-series crossover story arcs as "X-Cutioner's Song", "Phalanx Covenant" and "Age of Apocalypse".