monomania

Excessive interest or concentration on a singular object or subject.

It was apparent to all but himself that what was once idle curiosity had become a monomania.

A pathological obsession with one person, thing or idea.

“There are no limits to the possibilities of monomania,” I answered. “There is the condition which the modern French psychologists have called the ‘idée fixe,’ which may be trifling in character, and accompanied by complete sanity in every other way. A man who had read deeply about Napoleon, or who had possibly received some hereditary family injury through the great war, might conceivably form such an ‘idée fixe’ and under its influence be capable of any fantastic outrage.”