Doctor Strange indeed comes face-to-face with Shuma-Gorath, but it is a painfully familiar face contorted by primordial evil into a perverse distortion of the Ancient One's visage.
In the dank caverns of Kaa-U, the master of evil explains to the mystic how he came to being in our universe.
To illustrate the totality of his power, Shuma-Gorath toys with Strange, showing him potential future events such as Strange's own death and a world carved in Shuma-Gorath's image.
As Shuma-Gorath's existence in this plane of reality stems from the Ancient One's mind, Strange seizes on a desperate gambit and alters his size to microscopic proportions to enter the aged master's brain and oppose Shuma-Gorath at source.
Reality as one conceives it, warps to Shuma-Gorath's whimsy as Stephen Strange encounters old foes in the memory processes of the Ancient One. Further need for artifice is eschewed in this mental amphitheatre, and Shuma-Gorath reveals his true monstrous form.
An onerous decision is reached. In order to save the universe from the corrupting influence of Shuma-Gorath, Doctor Strange must kill his master, the Ancient One, destroying the foul fiend concurrently.
Without hesitation or sentiment, Strange destroys the ego of the Ancient One, therefore obliterating Shuma-Gorath. The caverns of Kaa-U crumble around Doctor Strange and his deceased mentor. Strange hardly notices, lost in his introspective guilt.
Outside the ruins of Kaa-U, the mage makes an astonishing discovery. The Ancient One communicates through the trees, water, rock and even the air. In his mortal body's passing, the Ancient One's essence has become One With the Universe. In his place, Doctor Strange becomes the new Sorcerer Supreme.