The servants of Zarriel once stole from Tiamat's treasure vault — no one knows how those guys managed to do it, and this has caused Zarriel and Tiamat not to have a harmonious relationship.
The opportunity to disgust Zarriel appeared, and as the divine chosen, Akhan would never miss it, using it to please his master.
Just while Zarriel is busy with the demon invasion, Akhan takes the chance to expand some territory, disgust Zarriel, and doesn't need to report to His Majesty Tiamat; as the divine chosen one, Akhan can make the decision himself.
Every Abisha was once a mortal who won Tiamat's favor in some way, and after their death, as a reward, these people's souls were transformed into dragon-shaped devils and continued to serve Tiamat.
