Legion

Legion,  a.k.a. David Haller,  has a severe mental illness including a form of dissociative identity disorder in which each of his personas controls one of his many powers.  He is an Omega Level Mutant with virtually unlimited powers.

First Appearance:  New Mutants #25 (1985)
Creators:  Chris Claremont,  Bill Sienkiewicz
Powers:  Omega Level Mutant

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Issues

Black entries are ongoing series
Green entries are Limited series
Red entries are One-Shots
Orange are alternate universe stories
Blue is for comments

Reading Order

New Mutants #26
New Mutants #27
New Mutants #28

Read Muir Island Saga here.

Read Age of Apocalypse here.  Only the Legion Quest prelude event is necessary,  reading the rest of Age of Apocalypse is optional.

New Mutants Vol. 3 #1 (2009)
New Mutants Vol. 3 #2
New Mutants Vol. 3 #3
New Mutants Vol. 3 #4
New Mutants Vol. 3 #5

Read Age of X here.

X-Men: Legacy #250
X-Men: Legacy #251
X-Men: Legacy #252
X-Men: Legacy #253

X-Men: Legacy Vol. 2 #1 (2013)
X-Men: Legacy Vol. 2 #2
X-Men: Legacy Vol. 2 #3
X-Men: Legacy Vol. 2 #4
X-Men: Legacy Vol. 2 #5
X-Men: Legacy Vol. 2 #6
X-Men: Legacy Vol. 2 #7
X-Men: Legacy Vol. 2 #8
X-Men: Legacy Vol. 2 #9
X-Men: Legacy Vol. 2 #10
X-Men: Legacy Vol. 2 #11
X-Men: Legacy Vol. 2 #12
X-Men: Legacy Vol. 2 #13
X-Men: Legacy Vol. 2 #14
X-Men: Legacy Vol. 2 #15
X-Men: Legacy Vol. 2 #16
X-Men: Legacy Vol. 2 #17
X-Men: Legacy Vol. 2 #18
X-Men: Legacy Vol. 2 #19
X-Men: Legacy Vol. 2 #20
X-Men: Legacy Vol. 2 #21
X-Men: Legacy Vol. 2 #22
X-Men: Legacy Vol. 2 #23
X-Men: Legacy Vol. 2 #24