Sythorr was a giant Haluu plant.
He escaped his race when some of the Haluu were against overgrowing worlds, eating "fleshlings" and waging war. He escaped in a dome-shaped escape pod containing a Garden, which landed on Titan and hid inside its caverns for centuries until humanity could master space travel so he could be rescued. He was the progenitor of a garden of Haluu when he stole the minds of humans through the Dreamspace as they died in their beds and wiped their memories, putting their "souls" in the garden. He chose Oksanna as his love.
When Oksanna called a human expedition to Titan, many of them were fed to Sythorr. Sythorr intended to wipe the memories of Bill Potts and Rudy Zoom and have them carry his seeds to Earth to spread his influence across the planet and overgrow it. After being reminded of her old life as a human, as an act of atonement, Oksanna destroyed the domed garden using the robotic Keeper's self destruct mechanism. Using the last of his life, Sythorr warned the Twelfth Doctor that "the unknown soldier...is stirring." (COMIC: The Soul Garden)