The Shadow is a fictional character published by Street & Smith and created by writer Walter B. Gibson. The enigmatic character originally served as a narrator for the radio program Detective Story Hour, intended to boost sales of Street & Smith's Detective Story Magazine. The protagonist was later developed into a distinct literary character in 1931, featuring in several forms of media, including comic books, comic strips, serials, video games, and films. As a crime-fighting vigilante, The Shadow was known for his expert detective skills, mastery of hand-to-hand combat, and ability to become invisible. The character's catchphrase, "Who knows what evil lurks in the hearts of men? The Shadow knows!", spoken by actor Frank Readick, became a recognizable idiom in American culture.

The radio drama, titled The Shadow, aired from 1930 to 1954 and was broadcast on networks such as CBS, Mutual Broadcasting System, and Blue Network. A total of 677 episodes were produced, while only a small portion, around 275, are known to still exist today. Throughout its run, the show was sponsored by companies like Blue Coal, Goodrich Tires, and Grove's Bromo Quinine, and featured episodes with distinguished voice talents such as Orson Welles.

Sources: wikipedia.org