A fictitious Babylonian deity found in Frank Peretti's first children's novel, Door in the Dragon's Throat, eventually identified by the author as the Judeo-Christian Satan. The name "Shandago" is not actually found in any Babylonian (or Sumerian, Assyrian, or Persian) written records - unless Mr. Peretti has access to artifacts that are not known to the scholarly community.
There isn't a well-known character or concept specifically known as "Shandago." It could be a unique name or not widely recognized term.