Garsennon

The Doctor was referred to as the "last tree of Garsennon" by the Testimony. (TV: Twice Upon a Time)

Behind the scenes
In the story Perfect Authentic Cadence by Charles Whitt, released in the unlicensed short anthology Sh*t Trips Volume 2.5, the Gestalt was established as a future of the Doctor arising after the "Last Battle of Garsennon" made them unable to regenerate normally. The Gestalt would go on to appear in a licensed context in Cwej: The Series, which contained vague hints about their past identity as the Doctor, but did not elaborate on the Gestalt's origins.