Sombra needs to complete only one last mission before he gets to retire once and for all and live out the rest of his day with his family.

The mission was simple: get in, secure the crystal heart, and subdue the earth pony princess. Seemed easy enough.

However, a mission of this sort done outside of Celestia's supervision is bound to have complications.

Sombra might should have waited for Celestia to dismiss him as a result of old age.