HDAC9 and cancer: While its roles in nuclear factor erythroid 2-related factor 2 (Nrf2) activation and histone deacetylase (HDAC) inhibition are well-established, this review provides a novel synthesis by integrating disparate research scales—a multiscale perspective that spans from the genetic and epigenetic regulation of glucoraphanin biosynthesis in plants to SFN’s recently elucidated effects on ferroptosis, cancer stem cells (CSCs), and the tumor immune microenvironment in humans.