Shiltzen is a gnomish alchemist-priest. He was rescued as an orphan by the elves, and brought up by them. For many years he travelled the lands, seeking knowledge, until he came upon some old prophesies, one of them concerning the firstborn of the Lord of Kaer Carreg, that 'only the firstborn of Jarl Harald can ensure the survival of the original inhabitants of Skellige'.
Determined to repay his debt to the elves by preventing their extinction, he travelled to Kaer Carreg, where he discovered jarl Harald dying on a hillside. As soon as he established who Harald was, Shiltzen made a deal with Harald: to save his life and help kill the monster that struck him, in return for his firstborn. Faced with no choice, Harald agreed.
When Shiltzen came to claim Harald's firstborn (Astrid), Harald gave him instead the monstrous Hafnir, the second of the twins to be born. Shiltzen was faced with a dilemma. He knew the custom was to leave deformed babies in the hills to die, but determined to see the prophecy fulfilled, Shiltzen took Hafnir in. It wasn't long before Shiltzen began to doubt the prophecy. Hafnir was a clearly a monster; a monster very like the one that Shiltzen had helped slay.
What's more, Hafnir grew astonishingly quickly; he was man-sized in three years, and continued to grow beyond that. But is was also clear Hafnir was intelligent and sensitive, and Shiltzen grew to love him in his way. Shiltzen remembered his own lack of parents and his upbringing by the elves. He was kind to Hafnir, and worked hard to ensure Hafnir's upbringing would be calm and as close to human as possible.
He invited Astrid to the house, so that Hafnir could see his sister, even though he was never allowed to meet her. But Shiltzen also saw Hafnir's dark side, especially his temper; so he kept Hafnir always 'penned' in, unable to go where he pleased, until one dark day Hafnir broke out of the pen, killed several elves and went into the wild. Shiltzen spent several months looking for him, but finally gave up after discovering no sign of Hafnir.