The phrase "spawn of Satan" is a popular idiom used to describe a person or thing that is evil or wicked in nature. However, using such strong language may not always be appropriate or polite. Fortunately, there are several synonyms one can use instead of "spawn of Satan" to convey the same idea. Examples include "demon spawn," "devilish offspring," "hell-born," "infernal creation," "malevolent creature," "satanic progeny," "sinful spawn," or "wicked prodigy." Using any of these synonyms can add variety and depth to your vocabulary while still conveying the intended meaning.