Gaia/Chronology

Majin Behind the Scenes arc
The corrupted dragon was unsealed by Free Guild Grandmaster Yuuki Kagurazaka after discovering its location through the Guild's network. It was placed under Mariabell Rosso's control and further agitated under the influence of her Unique Skill Greed. It was then sent to eliminate Demon Lord Rimuru. Milim, who accompanied him in their exploration of the ancient ruins of Amrita, was shocked at the unexpected appearance of the dragon, and engaged it once more to prevent its rampage.

Overpowering it, Milim destroyed the dragon's spiritual and astral bodies, and its heart core was devoured by Rimuru. In a show of generosity to Milim, Rimuru patched its broken heart core and placed it in a Pseudo Soul. It was fused with a Master Core, creating an Avatar which would serve as its body, reviving in the shape of its choice.