Arcata accepts the offered drink, but doesn't drink any right away. There are more important things on her mind...until she notices the change in Cassie. Of course, it might just be very good drink, or she might be the sort of person who warms quickly. But none of those things rang true in her mind. She didn't need a cantrip to know when things were amiss.
"We came not just to re-establish ties," Arcata says, holding the cup as if she was about to drink from it, then lowering it at the last moment. As she did, she watched Haxmar intently to see if he reacted. "but also to give you warning. Though beastmen had overrun the Keep's walls, it was not beastmen who defeated its last holders. They were led by a cunning creature. One of the fey; a creature that can take the appearance of its victims. When we retook the Keep, this fey managed to flee the slaughter of its minions."
She gestures with the cup, almost taking another drink again, but seems too distracted by what she's saying to do it. "When I learned of your fortress here, I realized that they might try something like that to you. It's why I brought our good Brother along. There are ways to ferret out dark fey magic, and reveal all guises, all tricks, and all treachery they can wreak."
"As the wretched fey are too cowardly, and honorless to fight openly, these ways are our only way to strike at them. With respect, I offer you our aid in rooting out any such efforts by them to invade your hearth and home. We must stand together, or they will defeat us one by one."
As she speaks, she continues watching Haxmar; trying to gauge how he's taking her words. She's dialed up the rhetoric considerably past where she would normally have spoken...trying to provoke a reaction of some kind. If this was really Haxmar, even beguiled by fey charms, he would most likely still believe himself to be the enemy of the fey and react as such. If this was another pretender, then its reaction might well be very different. Arcata wasn't sure if fey would care about being accused of cowardice and lack of honor, but if nothing else it should prove instructive. She was now very glad she'd not revealed her spellcasting, and now had a chance to imply strongly she had no such ability. Surrounded by a fortress of potentially hostile dwarves, they would need every edge if things went wrong.