ShadowKeep

Start with Shadowkeep, our first major indication of Savathun's plan. The Scarlet Keep is created by the Hive on the moon. During th expansion, we are tasked with fighting Nightmares: memories of former foes projected by a Pyramid ship hidden under the moon's surface.

So, let's start at the title screen.

Unsurprisingly, the ShadowKeep itself is Savathun's plan. The Scarlet Keep was engineered to siphon Pyramid energy. The contruction was overseen by Hashladun, Daughter of Crota. Following the death of Hashladun and her sister Besurith the Hive would begin sacrficing Nightmares to attempt to communicate with the Witness.

Terroris Somnum

When we first encounter The Nightmare of Crota, we are given an instance of Savathun's Song. We learn from the Hive Books of Sorrow that Savathun encouraged her nephew to create a tear in the fabric of his father's Throne World, leading to a Vex incursion, Crota's banishment and the taking of Quria Blade Transform.

While we have evidence of Savathun's influence in Crota's life, this would have the effects of sparking Eris Morn's desire for vengeance and tearing Toland the Shattered from his corporeal body.

Eris' quest would lead her to us. We would kill Crota for good, bringing his father to our solar system to enact vengeance on us.