Thanks for the chapter!
Well let's look over the suspects assuming the traitor is present. Fabiola seems scummy enough but doesn't seem the type to put himself at risk and that's assuming he could order his slaves to murder someone I doubt they could do it so cleanly, especially without feeling guilty about it. Besides, he'd more likely be spending his time indulging himself when not fighting. Relic doesn't seem to have the personality to murder somebody along with seeming very timid. Also, his group seems made up more of alchemists rather than spell casters or fighters considering the group name Panacea which is a miracle cure made with alchemy along with the fact that they don't seem to have weapons like a staff or sword. Although his face, expression, sunken baggy eyes, and sweating gives me the impression give me the feeling he's on something and hasn't had a fix in a while. Gale gives me the impression (especially with that paunch,) of somebody looking for a quick buck with minimum effort, and first to turn tail when things go south so would consider murder too risky. Eksvain seems like a by the book loyal subordinate who wants to accomplish his orders quickly for his kingdom with all the honor given for a job well done, so wouldn't delay things by murdering people or some lame attempt to get more rewards. Micaela and Ryuin didn't speak up and are more like background mob characters so not likely them unless do something more in the story than just being present. Considering how arrogant and dismissive Levia is towards everybody else she might do it on a whim or irritation although her party despite being scum who'd sell their friend into slavery while laughing don't have the right weaponry and would need some high reward to be worth the risk if get discovered. Then again, could be no traitor and somebody from a third party working from the outside trying to delay things or gets kicks from hunting adventurers. Or a monster with high intelligence inside the castle is doing ambush tactics and is somehow able to train or lead an elite group of monsters that can somehow emulate class abilities. Then again, considering monsters have a kill or be killed relationship with adventurers is there anything saying the lucky ones who won didn't steal the class stones thinking they were pretty or something and somehow got them to work?
In any case, lot of potential possibilities and suspects but probably won't find anything about the adventurer hunter, traitor or not, until ten chapters after Eleanor gets her revenge as that's the MC's main focus currently.