It's not a wine bottle, it's a mug of ale. It's inside a box in the crusader camp.
It was bugged for me though, either I gave the ale and I couldn't talk to the other guy to beat him and find his reason to be there, or I fought him and couldn't give the mug of ale to Kava anymore.
I don't think thats a bug. If you start a fight with them I think you got on their badside .
No, you can agree to fight him(it stops before killing them) to prove your strength, that way he explains why he is there, and you can finish the quest, but then you can't give Kava the ale.
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It was bugged for me though, either I gave the ale and I couldn't talk to the other guy to beat him and find his reason to be there, or I fought him and couldn't give the mug of ale to Kava anymore.
I can see where the confusion stems from, however. I'll try to think something up how to best address it.
At no time 'o day they're at the spot they're supposed to be - nor at any time of night.
In the end I was able to resolve it by first doing quests within Bridgefort and then later the assault on the crusader-camp.