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 slayed 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 lead 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 that Ren was tricked into killing the dragon emperor located in Dragon Village by the locals so they could attain the treasure from the dragon's layer after it was slayed however he's unaware of that fact.

Ren fell into deeper depression when he learnt the dragon he slayed was the adoptive father of Wynida and he was the reason why she was sold into slavery.

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.