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The quest becomes available after completing the first 4 steps of the Operation Elbrus Quest. After talking with Saladin Forge in the HELM, Empress Caiatl will show up at the War Table and brief you on the mission. She will then give you the Exotic Quest named Kill the Messenger. When the quest has been completed, you will be rewarded with the Dead Messenger Exotic Grenade Launcher. When the Master variant of this mission is cleared, you will be rewarded with the Dead Messenger Exotic catalyst.
The Exotic Quest takes place in Meridian Bay, on Mars. Caiatl's Psions have intercepted a Psionic broadcast that is persuading those with Psionic abilities to defect from Caiatl's Cabal and join their Psion conclave, which is allied with the Black Fleet. Caiatl says that she cannot be seen fighting her own people, so she sends in the Guardian to deal with the Psionic propaganda and shut it off. Zavala accompanies the Guardian on the comms while Amanda Holliday provides air support around the bunker where the broadcasts are coming from.
The Guardian fights past Cabal forces around the bunker, forcing their way inside until they eventually reach the source of the Psionic broadcasts, where they find Qabix, Insurgent. Upon killing the Psion, the Guardian stops the Psionic broadcast. After this, the Guardian finds a treasure room containing artefacts from the Leviathan (ship), containing the Dead Messenger Exotic Grenade Launcher. Upon claiming the Dead Messenger, Caiatl utilises Psion Cabal technology that allows her to see the Psionic broadcasts. She describes how she sees the silhouette of a Psion leading a Cabal Legion. She says how they are crowded around a golden statue of Emperor Calus, and behind them is a black Pyramid. She talks about how Calus's hands are outstretched, how his voice is promising her refuge, and promising her Salvation.
Despite clearing out the bunker in the previous week, the Vanguard and Cabal coalition discover that the base has been rebuilt and restaffed in the next, almost as if no assault had taken place, and the broadcasts have resumed. Caiatl presumes this is a result of interference from her father, Calus, who must be using his cloning facilities upon the Leviathan to continue the militant actions of the insurgents. Week after week, the coalition clear out the base, discovering new layers to the Psionic broadcasts. Caiatl witnesses the Last City besieged, Eramis revived from her frozen prison, the Leviathan reborn and the Traveler itself infected with Darkness.