The Siege of X-41 : A Marvel: School of X Novel

Omega-level mutant Elixir must come to terms with his anti-mutant upbringing while fighting off vampiric mermen and a leviathan of ancient lore in this intense survival adventure from Marvel's School of XOnce, Joshua Foley hated mutants, then he became one. It's... problematic. When his healing powers manifested, the violent anti-Mutant Reavers rejected him and only the X-Men offered sanctuary. Now he's surrounded by mutants who still see him as their enemy. A deep-sea training mission isolates him with a select team of new X-Men, which is bad enough, but then the attacks begin. A sect of sea monster-worshiping vampire mermen besiege the base, leaving them stranded and trapped. As the attacks escalate, their chances of survival plummet. Things go from awful to even worse when one of their team begins sabotaging their attempts to call for help, all in the name of a sleeping behemoth soon to awake...