The phrase "dark purpose" can refer to intentions that are sinister, malevolent, or malicious. Some synonyms for this phrase could include "evil intent," "nefarious design," "wicked aim," "sinister motive," "malevolent plan," or "malicious agenda." These words all convey a sense of something dangerous or harmful lurking behind the author's words. Other synonyms might include "shadowy aim," "iniquitous goal," "perfidious ambition," "villainous objective," or "diabolical scheme." Writers might choose to use these alternate phrases to convey a more nuanced or complex sense of malice or evil intent in their work, rather than relying on the relatively straightforward "dark purpose".