His ability, The Great Old Ones is a reference to Cthulhu's origins; a 'Great Old One' is a God of extra-dimensional origins. However, it is not actually an ability. It is heavily implied by his dialogue about a "contract" with Francis Scott Key Fitzgerald that he actually is the "Great Old One" he transforms into, particularly due to him disappearing into the sea at the end of The Guild Arc.
(From Bungostraydogs Wiki)