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The Mirage Jar is an item found in Level 155.
It is a normal and ordinary glass mason jar. It can contain any item that can fit in it.
However, in exchange of an item being placed into the Mirage Jar, an entity called the Mirage Jar Spirit will form and will linger within the jar. This can lead to harmful side effects such as psychological trauma and hallucinations.
Regardless of how much is placed into the jar, the spirit will still linger and will feed off of your emotional strength. It doesn't manifest to physical harm.