Spore-Bound Essence
Drops from Spore-bound Essence. The Overgrown Skeleton can be tracked with undead tracking abilities - such as Track Undead on a Hunter, or the elixir from the AH.
Spore-Bound Essence is a toy dropped from an Overgrown Skeleton in the Zskera Vaults on The Forbidden Reach, obtained by completing a small puzzle involving alchemy supplies found nearby.
Guide awaiting review
Reach Layer 2 of Vaults
The Overgrown Skeleton can only appear on the 2nd Layer of the Zskera Vaults, so progress through the vault until you reach that level.
Locate the Skeleton Room
Find the room filled with bone piles — the Overgrown Skeleton NPC sits at the back of this room. The skeleton is easy to miss, so scan the back of the room carefully before moving on.
Use Undead Tracking
If you have a Hunter with Track Undead, or an undead tracking elixir purchased from the Auction House, use it to help detect the Overgrown Skeleton without needing to open all Layer 2 rooms.
Collect an Empty Vial
Find the nearby alchemy room filled with empty vials and pick up at least one 
Empty Obsidian Vial from inside it.
Fill the Vial
Use the Empty Obsidian Vial on the water source located in the same alchemy room to fill it, creating 
Restorative Water.
Use Water on the Skeleton
Return to the bone piles room and use the Restorative Water on the Overgrown Skeleton. Mushrooms will grow out of the skeleton, which you can then loot to receive the Spore-Bound Essence toy.
Tips
The Overgrown Skeleton room is not guaranteed to appear every vault run — the room is subject to RNG, so you may need to clear the Zskera Vaults across multiple weeks before encountering it.
