We had a situation in Talisman 4th Edition Revised where a player got the Warlock Quest "Take One Life From Another Player".
"Seek the one with the mark of the Talisman - a mark that only you will see. When you find him, vanquish him in combat. His blood shall be your key. His suffering shall be your salvation."
The trouble was that the character wasn't allowed to attack another player. We were playing with the Lightbearers alternative ending that disallows player versus player combat, but this could also happen if, for instance, you are the Knight and all other players are Good for the time being (or worse, they are characters like the Knight who are permanently Good), since the Knight can't attack Good alignment players.
Is there any way to complete this Warlock Quest if you can't attack other players?