From Destinypedia, the Destiny wiki
During the Dark Age, when Zavala was a member of the Iron Lords, he had fallen in love with a woman named Safiyah, a wandering chirurgeon that stayed with their camp. One day, when the two were searching a caravan attacked by the Fallen for survivors, they heard an infant's cry. They found the baby clutched in the hands of a now dead man, so tightly Safiyah had to break the corpse's fingers to free it. They brought the baby back to the Iron Lord's camp, where they decided to name him Hakim. Months later, the family would leave the camp to go live in the village of Safiyah's sister Amani.
As the years passed, the three would continue living as a family, while Zavala taught Hakim how to fight, and Safiyah's continued her chirurgic and motherly duties. When the boy was fifteen, Zavala wanted to teach him how to take a life, as it would be needed for when the Fallen would attack, by having Hakim attack him with a knife. Safiyah had an argument with her husband and son over this, though she knew it would be something he'd need to learn eventually.
When Hakim was seventeen, the Fallen suddenly attacked the village. Zavala immediately rushed to fight them, while Safiyah prepared to intervene on the wounded. Hakim wanted to take part in the defense, but his mother asked him to stay home, safe, as she left. Hakim, however, joined the battle anyway, and was eventually confronted by the leader of the raid, Kethiks, the Yet-Proven. The young man fought valiantly, but was no match for the Captain, who unbalanced him with a strike and then impaled him on his Arc Spear. Kethiks would die moments later by Zavala's hand, but Hakim's wounds were too severe, and he died in his mother's arms soon after. He was later buried in the village's graveyard.
Hakim's death shook Zavala and Safiya to the core. The Guardian even threatened his Ghost Targe with his gun to try to force him use his own Light and bring back his son, despite knowing it was impossible. He may have pulled the trigger too, if Safiyah hadn't stopped him. Ultimately, the couple split, with Zavala retuning to the Iron Lords, and Safiyah eventually finding love again, having a daughter, and dying of old age. Zavala would continue to blame himself for the death of his son, for instilling in him too much bravery, but kept his grief a secret from almost everyone.
Centuries later, Zavala's past would return to haunt him, with the return of the Leviathan. The ship started connecting itself with The Lunar Pyramid, drawing numerous Nightmares to it. The Nightmare of Safyiah would taunt the Commander with the memory of his son's death, trying to shake his convictions, and causing him to fail the first severance ritual. However, after a talk with Amanda Holliday, he gathered the strength to forgive himself, and complete the second severance ritual, transforming the Nightmare of Safyiah into the Memory of Safyiah.