In the most unforgettable picture in this thrilling show, the Belgian artist Léon Spilliaert stands in a dimly lit room and stares straight at — or through — you. It is difficult to be certain because his eyes are shrouded in darkness, just washes of black. In a neat suit, with hair aglow, he is unworldly. It was 1908, he was 27, largely self-taught and ambitious, an illustrator now creating symbolically charged works on paper with ink, watercolor, pastel and the like.Spilliaert lived in the seaside city of Ostend,...