The Apparatus is located in the jungle biome , directly east of the Rumble Ruins.
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The Apparatus is a ruined structure that can not be entered, but it is a home to a number of secrets including a secret quest that rewards you with 20,000 XP.
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To find the Apparatus , start at the Rumble Ruins landmark and head east along the river.
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The Apparatus will be an your left, just before you reach the Creeky Compound landmark.
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Once you find the Apparatus, climb to the top of the structure to find Trace, a secret quest.
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The quest requires you to find a Prism and bring it to Trace. The prism is located in a temple that is just north pf the Apparatus.
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Once you have the prism, bring it to Trace and he will give the XP reward. You can also trade with Trace for gold bars or weapons.