Looking at both stories in question, I can't but notice similair aspects from them:

Both characters are found with no memory of their past.

Both are the remains of a dark entity that was a threat to Equestira.

Both befriend a foal from their new home.

Both try (and in Sombra's case fail) to keep themselves from becoming what they once were.

Both are confronted by the Princesses who try to convince them not to follow their dark path and imprison them.

Am I the only one seeing this?

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This isn't all that uncommon of a plot. It's used more often than you'd think to develop interesting heroes or villains, allowing the readers to grow and share in new experiences with the character being focused on.