A Storm God is a giant weather-shaping machines used alongside Lovelock engines to terraform a planet.
It's commonly known that the only Storm Gods left are on Triton and Pluto. However, on Mercury, the Storm Gods were buried and hidden under the order of Octavia au Lune, who kept their existence a secret in case she ever needed to use them against the ruling family on Mercury, House Votum. As her grandson and heir, Lysander au Lune knew about the Storm Gods. Darrow and Virginia au Augustus also know about them after they managed to break into Octavia's Crescent Vault after her death.
Darrow activated one near Tyche on Mercury in order to throw off Atalantia's troops, however, Orion disobeyed Darrow's order to keep it at Level One, which destroyed the city and killed millions of citizens.