Mordred stood behind Morgana and watched as the Druid, Ari was brought before her. He had no idea exactly why Morgana had summoned him of course or he never would have come. He was a powerful man and someone Mordred thought they would want to keep as an ally. He agreed that if they were to be successful in their plan to take Camelot, Emrys needed to be disabled. However, he did not agree with her plan to test the gean canach on him.
As he listened to the exchange between Morgana and Ari, he figured Ari must assume he was there to display his great power. The Druids were used to hiding their magic from others and of course protested using it for show. Ari was no different.
After displaying his ability by extinguishing the torches, Morgana summoned the men to bring forth the box containing the gean canach.
Ari recognized the symbols, panic and fear evident in his face. Morgana ignored him, unleashing the creature to feast on his magic.
Mordred stood firm, keeping his face hardened and impassive. But as he watched, he couldn't help but doubt. Was this display really necessary? He and Morgana both knew the powers of the gean canach to be true. Why the need to test it on anyone other than Emrys?
After Ari was escorted out, he questioned her decision. Morgana was adamant that it was necessary, whatever the cost. She was sure they now had the key to winning the war.