Magnus is the child of Princess Florimel and General Lucius Grayloque.  The two regret to this day that they didn't meet sooner, but fell in love almost instantly and were married in a lavish and almost unparalleled ceremony that swept the city like a storm.  Grayloque is immensely popular with the military and a decorated war hero.

Magnus was born several years after the wedding, and, as the child of a renowned beauty and a handsome, dignified gentleman, astonished everyone who was expecting him to be the perfect goldenhaired child.  Signs began when the pregnancy, which had till then seemed normal, culminated in an extremely difficult and painful birth process (caused by the ratio of Magnus' rather large head).  Flora was swept off to the hospital and the baby was delivered by Caesarian section.  He was born with achondroplasia, a form of short-limbed dwarfism.  Immediately, Flora combed Shadow for promising genetic treatments, but Magnus' Amberite DNA was too strong.  By late childhood it was obvious that nothing would cure him because his genetics believed there was nothing to be cured.

Beyond the physical, however, Magnus was a wonderful child.  Even as a baby, he was calm, serious and gentle.  He watched everything, and developed a strong sense of touch, texture, color and taste very early in life.  He adored physical contact and loved to be held, and despite some early misgivings, bonded with both of his parents.  His mother was first to get over her disappointment and would constantly have her son in her lap, talking with her, when they were at home.  Flora made the decision to limit Magnus' forays outside their home in his early life, since he was sensitive and not comfortable with new things, and since casual comments about his condition confused the child and infuriated her.

Magnus was quiet and introverted, entirely happy in the world provided by his parents, and fond of structure and responsibility.  If his father had any lingering doubts, they were swept away by Magnus' serious attitude toward learning, toward schoolwork, chores and being given more responsibility at an early age.  He also loved making new friends, although he was initially shy.  He loved to help, loved animals and adored the few friendships he made.  He was easily hurt by those who made fun of him or stared at him, and while probably his parents ought to have involved him in more events and activities so as to toughen his skin early on, neither of them enjoyed seeing their baby in tears and so he had a very small, very tightly-knit group of people he interacted with early in life.

Given that he is post Patternfall (but not by much, it's where his parents met), he'd be about three or four years younger than how much time has passed since the end of the Corwin chronicles.