Massive black holes (MBHs) are the brightest, most massive, and most energetic sources in the Universe. How they form and grow in the early Universe, and how they can transform their host galaxies, remains a mystery. Following galaxy mergers, pairs of MBHs can form binaries which produce low-frequency gravitational waves (GWs) that will soon be detectable by pulsar timing arrays, like NANOGrav. In this talk, Luke Kelley will describe his efforts to develop the most advanced and comprehensive simulations available for studying MBH binaries. He will share predictions for both GW and electromagnetic signatures and highlight the unique discovery space opened up by low-frequency multimessenger astrophysics. In addition to constraining the co-evolution of MBHs and galaxies over cosmic time, NANOGrav will also serve as a crucial test-bed for the future space-based LISA mission.