“Following the abrupt end of the Lucent Hive psychic analysis program and Lord Saladin’s integration into Empress Caiatl’s War Council, the Vanguard discovers that the Leviathan, the flagship of the exiled Cabal Emperor Calus, has suddenly reappeared in the Solar System in orbit above Earth’s Moon and is attempting to commune with the Lunar Pyramid. In response to Emperor Calus’s return and the threat from his newfound allegiance to the Black Fleet and the Witness, the Vanguard sends the H.E.L.M., fully revealed to be a mobile command ship, to the Moon’s orbit, and the Guardian is then tasked by Commander Zavala and Eris Morn to investigate the Leviathan. As the Guardian explores the now derelict Leviathan, they discover that the ship has been overrun by Darkness and infested with Egregore, Darkness fungal growths that were previously seen on the Glykon during Season of the Chosen, as well as Nightmares generated from the Lunar Pyramid.
In the H.E.L.M., Eris explains to Caiatl about the Nightmares infesting the Leviathan and that she is able to provide protection through a Hive séance ritual involving the Crown of Sorrow, now under Eris’s control, and an artifact of her creation called the Nightmare Harvester, but Caiatl refuses to get involved due to Eris’s usage of Hive magic. Eris then performs the séance ritual with Zavala, Crow, and the Guardian to bind them to each other and to the Crown of Sorrow, which also manifests the Nightmares of Safiyah, Uldren Sov, and even Dominus Ghaul that begin to haunt Zavala, Crow, and Caiatl. The Guardian is then given the powered Nightmare Harvester to help contain the Nightmares haunting the Leviathan and bind them to the Crown of Sorrow.
Working with Eris, the Guardian sought out ways to weaken Calus’s connection with the Pyramid and ways to transform the bound Nightmares into memories. One by one, Zavala, Crow and Caiatl found the strength to overcome their past and successfully completed further rituals to transform the Nightmares. During this time, fireteams of Guardians also met up with Eris in the Leviathan Tribute Hall where they were given the opportunity to enter the fractured mindscape of Calus to steal his darkest secrets.
Weeks had gone by and Emperor Calus had successfully begun to integrate his consciousness into the Lunar Pyramid. With time running out before Calus fully assumes complete control of the Pyramid, the Guardian, Eris, Crow, Zavala and Caiatl returned to the Leviathan to perform one more severance ritual, this time in the heart of the Pyramid where Calus’s consciousness resides, and successfully defeat Calus, though he is not truly gone. A message was later received in the H.E.L.M. from Calus, warning the Guardian that the Witness is coming for the Traveler.
During the season, Guardians participated in the Nightmare Containment on the Leviathan using a relic called the Reaper to cleanse areas of Nightmare manifestations. Eva also decorated the Tower in the summer for the annual Solstice Event which introduced a new Event Card system. Solar Guardians rejoiced as they found their Solar subclass to be reshaped into something new.”