In "The Crystalling" episode, Flurry breaks into a tantrum and completely shatters the Crystal Heart,
Not only did this cause the Crystal Empire to be covered in eternal ice and snow, it also secretly brought back someone else.

Before the Crystalling, Sombra and Luna where meeting (and dating) secretly for a couple years, nobody had found out til' Luna had secretly given birth to their daughter Neoma Artemis years later. The two had a rocky relationship from the start,basically like a "love/hate" relationship kinda thing,

Anyhoo, the two kinda distanced themselves every so often, however Sombra, being a shadow and all, always secretly followed her along the way, especially when the crystal heart was shattered by Flurry, Because although they had mixed feelings for eachother, he secretly adored and loved her dearly.

From the start, Luna had and was reforming Sombra, however, once Sombras hatred and fear went away, he became bitter and cold, emotionless to say the least. All he ever knew was hate and fear, what else could he feel now?