Uber Sombra full stop. While Sombra in general is pretty funny, his Split Personality is every bit the monstrous creature he was in the two parter.
Of special note is during Coffee and Sombra's making camp while going up the mountain. Everything's normal...then suddenly, Uber Sombra takes control and tries to murder her.
What really makes it terrifying is that there was absolutely no build up or foreshadowing to this happening. Uber Sombra can completely take Sombra over without effort whenever he wants, and there's (so far) nothing anyone can do about it! Well, no one except Sombra, anyway, as the two personalities need to be in agreement for Uber Sombra to do anything. But his sanity and the extent of his self-centeredness make relying on him an unreliable prospect at the best of times.
Plus, later Uber Sombra shows he's quite willing to dispose of Coffee Talk if he can do so in a way Sombra can't find out about.
When Sombra finds out that Supremus Longhorn and Dominus were attempting to control his mind, Sombra becomes so enraged that Uber-Sombra takes control and tears off one of Longhorns legs (and would have ripped off the rest off if Pun didn't stop him).