Wow, I somehow managed to complete this quest, since Tadeas absolutely refused to give me the serum. I was able to make the serum myself through alchemy by experimenting with different combinations.
I might not have explained the problem to you correctly, so here’s a small clarification: when I talk to Tadeas, our girl says that some kobolds were lying in ambush and stole the serum. And it doesn’t matter when it happens—right after accepting the quest or after checking the alchemist’s hut.
And about the Bloodstone — I was able to find it easily in the spider mine. I was just searching in the cave with the entrance near the beach, so maybe I was looking in the wrong place.






