desenmascarar

To unmask someone — remove a literal mask, or, very commonly, expose someone's true nature, lies, or hidden identity: desenmascarar a un impostor, desenmascarar una mentira.

• El periodista desenmascaró al estafador. — The journalist exposed the con artist.
• Al final de la obra, el héroe se desenmascara. — At the end of the play, the hero removes his mask.
• Quisieron desenmascarar la corrupción del gobierno. — They wanted to expose the government's corruption.

The figurative "expose the truth about someone" sense is by far the more common use in modern Spanish — the literal removal of a physical mask is comparatively rare outside costume or theater contexts.

Often confused with enmascarar.