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Original Description:
 
I discovered in The Magic of Cybertron so King Sombra believed Cybertronians and he said “Show me all the magic of Cybertron and watch me bend it to my will!” So the answer is simple. The fact is, Alpha Trion is the writer of the book history of Cybertron known as “The Covenant of Primus” while he is safeguarding to Equestria by a pony who trust him, Starswirl the Bearded. Since King Sombra discovered the private property to Starswirl, he stole the Covenant of Primus while he touched for the first time. So that is why King Sombra stole the Covenant of Primus so that he can learn about the history of the Transformers.  
So, if you watched in Season 9 while King Sombra is not dead, he turned into the spark and he alongside with Unicron, placed him into Umbrum Crystal and the Dark Energon. So King Sombra’s plan to make “Darkest Hour” to Cybertron using Dark Energon to poison the main core of the planet.