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The Slimeball is a vanilla item that used to drop only from slimes. Now, with the addition of this mod, they can also be obtained as a mob drop from trolls or by tickling them to make them sneeze. They are meant to represent a troll's mucus.
Since the slimeballs are not an added feature of this mod, but rather an implemented feature from vanilla Minecraft, they retain the same appearance and physics in Middle-Earth as they do in the Overworld.
Slimeballs are rather difficult to find for good players, because they must travel to troll infested regions such as the Ettenmoors or Trollshaws and kill trolls for them, because Slimes can't be found in Middle-Earth.
Evil players can go up to a regular troll and right-click with a feather in their hand a few times to make the troll sneeze, which gives you slime (and a great deal of laughter and satisfaction). Note that each right-click removes a feather from your inventory.
While mostly useless in Middle-Earth, the few recipes that include slimeballs are fairly important, mostly for the crafting of leads.
They can also be used to make sticky pistons, though due to the lack of redstone (an essential ingredient in crafting pistons) in Middle-Earth, a trip to the Overworld is required.