Some monsters were too strong for their level to be interested in ordinary battles and even valuable resources, they were like that for other monsters but not for special ones.
The Dormant Lord was one of them, the proof of that was not him ignoring the situation but not paying attention to Katrin.
Sure, the Dormant Lord noticed Katrin, he was aware of her presence as soon as she entered the Crystal Gorge.
He allowed her to roam his territory, to fight against his subordinates and kill them. If Katrin were to take and steal some energy crystals and wanted to run away, the Dormant Lord would not stop her.
Why then did Dormant Lord destroy the fighter that just appeared in his field of vision? Katrin was much closer, but the monster wouldn't attack her.
It was because Katrin was worthy of such treatment from Dormant Lord, he could sense it from her aura and because none of his monsters were able to harm her.