I think that the last item is about right in difficulty to make. Just around the time every one has a good hero/items. I get it, all the games I've been in no one knows about it.
You should put something in the quest log about it though.
Something along the lines of, "The three kings hold powerful artifacts. Kill them to be rewarded."
That way people know there's a quest to do
Just thought of something too..
What you should change is the drop rate of the kings swords to like 3-5% and make it so they drop more than once. So every one would be able to get the +5000 sword. But make the Armor/Gem only 1 available. So that way only 1 person gets the ultimate item, but other people can still get a pretty good item.
I think you should also make chaos drop something better than just a random item. Perhaps an item that can be combined with the ultimate item to make a better one as well as able to combine with the +5000 damage sword so people don't feel left out.. ya know what I mean. So it would have to drop more than once, I would make it have no stats or anything just an item but combined with the others it makes something.
just a few thoughts.