Zombie Dragon

The Zombie Dragon is the reanimated corpses of a dragon emperor still possessing one the Dragon Emperor Cores.

The owner of the deceased body belonged to a dragon emperor called Gaelion who was the adoptive father of Wyndia.

Volume 2
The dragon emperor was originally slain by Ren Amaki the Sword Hero before reanimating into the Zombie Dragon, after hearing news of a dragon causing havoc from the locals of Dragon Village.

After slaying the dragon, he left the corpse to rot causing it to pollute the land and spreading disease throughout the village. This eventually leads to the reanimation of the corpse.

Naofumi and his party had to later clean up after the mess the Sword Hero had left behind.

Volume 3
After learning of the Zombie Dragon incident and the curse burns Raphtalia had been inflicted, Ren apologized both to Naofumi and Raphtalia for the trouble he had caused.

He also explained the reason why he left the corpse behind leaving it to the villagers and any passing adventurers which lead to Naofumi lecturing him.

Volume 12
It was later revealed the locals of Dragon Village tricked Ren into killing the dragon emperor so they could attain the treasures from the dragon's layer after it was slain. Ren, however, is still unaware of that fact and only knows that the dragon he killed was the adoptive father of Wyndia who was sold into slavery because of his actions.

After the events of the Demon Dragon Emperor incident, the original Gaelion now shares the same body as Naofumi's pet dragon who shares the same name.

Trivia

 * Naofumi absorbed one of the three fragments of the Dragon Emperor Core into his shield. Because of this whenever he activates his curse series near the Sword Hero he feels the rage of the original Gaelion.