What he saw dazzled him, she was real but there was another her like a shadow with a massive head that vibrated a certain frequency, echoing through the land. Her real body was barely holding it at bay. That's why she acted like a robot unmoving as she sat. Now that she was moving, she held an item that chained the shadow to her.
"Hmm. Alright." Seth said as he turned off his sight before he added, "when we trust each other then!" The last part was an answer to her desire to have her aura activated. With that, she may be able to contain her ability.
She turned and moved away, now that he saw the real world, the 8 players all held weapons against him. His group also were looking at them ready for a fight. Only Allen was in the middle trying to talk things out.
Seth wasn't able to perceive that because the sight was dazzling and the sound was drowning. He would have had his eyes melt out and his brain boiled down if not because of Aura and Pure White Stone Will.