Akatsuki no Yona OVA 2 - The Yellow Dragon Zeno's Past, Part 1 The Starting Drag

The second and third episode covers Zeno's backstory and gives a glimpse of King Hiryuu and how Hiryuu castle came to be. Zeno's powers are also revealed in these episodes. They are a summary of the 21st and 22nd volumes of the manga and cover the Zeno arc. The heartwarming relationship between the first generation of dragons and king Hiryuu shines a light on why Zeno shows nostalgia when he meets the Dark Dragon and the Happy Hungry Bunch.