These are special objects that are created when a character fulfills a quest. These objects can perform various tasks for example heal Zombie's Rott, make a character look normal after he was deformed and so on. The quest crosses disappears after they have been used, some can be used multiple times, but after a few minutes they all disappear. there is no way to keep such a cross over a server restart, so you will only find those objects when you fulfill a quest or come across such an object after somebody else fulfilled a quest.
To use such a cross, just stand next to it and double click it.
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