Who is the Black Dragon in A Knight of the Seven Kingdoms?

A Knight of the Seven Kingdoms episode 5 doesn't just give viewers a brutal trial of seven to prove Ser Duncan "Dunk" the Tall's (Peter Claffey) innocence. It also offers up an extended flashback to his past as an urchin in Flea Bottom — and that past includes a Westerosi history lesson.

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After scavenging around a battlefield with his friend Rafe (Dune...