Complex sentence: Rockslide is the codename of Santo Vaccarro, a fictional character, a mutant in the Marvel Universe, one of the student body in the Xavier Institute and a member of the former Hellions squad therein.
Split into simpler sentences: Rockslide is the codename of Santo Vaccarro, a fictional character and a mutant in the Marvel Universe. He is a student in the Xavier Institute and a member of the former Hellions squad therein.