It usually takes twenty minutes on a horse or a carriage from the arena to Leoric's castle, but Artemis used just five minutes to fly from the arena to the castle where he found out there was a battle going on between his men and Leoric's remained Killers. Dante and Aeryn were yet to arrive, Artemi's men were losing the fight and that's when he knew he made a mistake bringing in five hundred men.
There were fifty Killers who were supposed to guard the castle in case anything happens, when Artemis arrived only forty of them remained but on the other side he'd lost two hundred of his men. The remaining three hundred men were lost and not knowing what to do until they saw Artemis landing in front of the castle, and the dragon screamed at the Killers as if it was ready to burn them right there.