Duncan became Ferelden's Commander of the Grey in 9:10 Dragon, assuming the mantle from Commander Polara, under whom he had served since the order's restoration in Ferelden.

Polara, who hailed from Antiva, had built an amiable relationship with King Cailan. She overcame many of the objections of Teyrn Loghain, who had argued strenuously against the order returning—after all, the Wardens had attempted to overthrow the Ferelden throne centuries before.

When Polara disappeared—perhaps recalled to Weisshaupt, although none in Ferelden saw her leave, nor have seen her since—Duncan bequeathed to her son a silverite axe of great value. Duncan said he had wielded it and a twin in younger days, back when he had been a far different man. He'd named the axe "Fiona" after a Warden who inspired him, and suggested that the son ought to take similar inspiration from his mother.

It's unknown what became of Polara's son, other than that he fled Ferelden during the same Blight in which Duncan perished.
—From Codex entry: Fiona