Shadowmist Assault was not originally an AoE skill, but the moment the targets' positions were perfectly coordinated, it was possible for it to become an AoE skill.
The row of Mages was very coordinated at the moment, lined up in a neat line, all waving their magic staves to call their fiery assault on Gu Fei and the three soldiers. They were relishing the sight of the four interlopers dying in front of them as they went through the motions of unleashing their spells. In the end, that gave Sword Demon the chance of using his Shadowmist Assault like an AoE skill.