Greed, anger, lust, jealousy, slaughter...
Each head of the giant turtle represents a different desire. Evil is born from human desires, so they naturally evoke the deepest desires in human hearts.
Thus, every glance, every twist of the giant turtle's body can stir the deepest desires of the soul.
Even though Xiao Yue and the others, who have passed away, hide within the Ten Thousand Soul Banner, they feel their spirits waver upon seeing this massive evil entity. Fortunately, they are protected by the Ten Thousand Soul Banner, as their divine souls and fan souls belong wholly to it, and it is merely Shen Siyuan's visual projection, so they were not mentally shaken.
If it were ordinary ghosts, even just watching Shen Siyuan's visual projection, their souls would likely be twisted by desire.
