X-Men: Dark Phoenix Saga [New Printing 2] by Chris Claremont

An epic tale of triumph and tragedy! The X-Men have fought many battles, been on galaxy-spanning adventures and defeated enemies of limitless might ― but none of it prepared them for the most shocking struggle they would ever face. One of their own, Jean Grey, has gained cosmic power beyond all comprehension ― and that power has corrupted her absolutely! Now, the X-Men must decide whether the life of the friend they cherish is worth the possible destruction of the entire universe! This touching tale of ultimate power and the triumph of the human spirit has been a cornerstone of the X-Men mythos for decades. Relive the saga that changed everything! Collecting X-MEN (1963) #129-137.