There’s an NPC in Valdrakken that lets you watch all 3 Dragonflight Legacies.
She’s called Storidormi and is found at 60, 42 (upstairs in the Seat of the Aspects).
You can watch the following Legacy cinematics:
Dracthyr’s origins // Chapter One
Neltharion’s Downfall // Chapter Two
Dawn of the Aspects // Chapter Three