Mysterious Lost Sands is the fourth chapter of Bug Fables: The Everlasting Sapling, in which Team Snakemouth learns that the artifact they had collected from the Honey Factory was only half of the Ancient Key, so they head back into the Lost Sands to uncover the hidden Sand Castle where the other half lies. It is preceded by Chapter 3: Factory Inspection and followed by Chapter 5: The Far Wildlands.
As Team Snakemouth exits the Honey Factory, Vi stops for a moment on the bridge. After Malbee and the Overseer changed their minds about her, Vi wonders if it's possible to patch things up with her sister too.
Kabbu asks Vi what kind of bee her sister is, and Vi answers that her sister is a painter in the Hive named Jaune. Leif says that if Vi wanted to go visit her sister, they wouldn't mind, since their mission is already complete. Vi says thanks, and that she will think about it.
The team arrives at Anthill Palace, and the Queen is joyous that they have finally found all of the artifacts. Zaryant rewards the team with a Crystal Berry and 50 berries. Upon further inspection of the artifact, Professor Neolith figures out that it isn't complete - it's just one half of the Ancient Key. The Queen is worried, as they are running out of time. This is because, as Zaryant explains, the Wasp Kingdom is growing bolder with every passing day, and will soon make their next move.
Team Slacker arrives just in time with the results of their scouting mission; the other half of the Ancient Key is hidden within the Lost Sands. To obtain it, two special keys are needed to reveal the Sand Castle, in an area previously inhabited by the roaches. The Mayor of Defiant Root, they say, knows the whereabouts of the two special keys. The Queen is grateful to Team Slacker, and wishes to reward them, but Delilah says they can be rewarded later, as they are going back out again to investigate further.
The Queen says that she would give this mission to Maki, but he still hasn't returned from his current mission, so she instead orders Team Snakemouth and Team Mothiva to begin their search. With the Bubble Shield field skill, Leif wonders if they can find new paths across the spikes in the Lost Sands.
In Defiant Root, the Mayor tells the team that to enter the Sand Castle in the northeastern part of the Lost Sands, they need the Heaven and Earth keys, one of which is with the bandits. As the team heads out, the Mayor generously gifts them 45 berries.
For the price of 30 berries, a suspicious, cloaked figure found at the bottom of the well in Defiant Root gives the team a Rusty Key, which they can use to unlock the Bandit Hideout. Once inside, the Earth Key is seen lying on a pedestal. However, when the team goes to take it, it turns out to be a trap, and a mob of bandits capture them.
The team wakes up in a muddy prison cell, without any of their items, including Vi's Beemerang. There seems to be no way out, but Kabbu gets an idea; he can dig through the soft soil, and they can escape the cell with his new dig field skill. Kabbu also learns the Under Strike skill.
Now out of the cell, the team can navigate through the hideout, but they will have to be stealthy, so they don't get thrown back into their cell again.
When they enter the treasure room, the team overhears a conversation between two wasps and two bandits. The wasps haven't paid the bandits like they said they would, even though they captured Team Snakemouth as requested.
In the treasure room, the team finds the Beemerang and all their other stuff, meaning they can engage in battles once more. The team finds their way back to the entrance, where they first got trapped, and by continuing down the hallway, they meet a cloaked bug.
The bug takes off his cloak and reveals himself to be Astotheles, the leader of the bandits. He was expecting a reward from the wasps, but it was all just a trick. Having no respect for the Ant Queen's explorers, Astotheles decides to fight Team Snakemouth, but is defeated.
Astotheles requests to be killed after his defeat, but Vi tells him not to be so dramatic, and that they only want the key. The bandits rush in, concerned about their injured boss. They want to fight Team Snakemouth, but Astotheles stops them. He sees no reason for the team to stay in the hideout any longer, now that the wasps are gone. After figuring out the team is against the wasps as well, he entrusts them with the Earth Key.
As they search the Lost Sands, the team comes across some Roach ruins, where they find Hawk. He had found the Heaven Key, and gives it to the team, with the hope that they find an artifact with it, which can boost his research.
By putting both the Earth Key and Heaven Key together, the Sandcastle Key is formed. The team decides to head northeast, towards the Sand Castle.
Unfortunately, on the way, a Dune Scorpion appears out of the sand and attacks them. Right when the team lets their guard down after they think the Dune Scorpion is knocked out, it wakes up, and badly wounds Leif, causing strange, white appendages to sprout out of him briefly. Zasp suddenly appears, to stop the scorpion, but he's too late, as the damage was already done.
Leif, while unconscious, hears voices talking about a failed experiment. Hours pass, and Leif wakes up. His wounds were patched up by Zasp. Kabbu and Vi are happy to see Leif is alive, as they were very worried. Despite his injuries, Leif decides to go with the team to the Sand Castle, where he hopes to find answers.
After inputting the Sandcastle Key, a gigantic dune rises from the sand, and reveals the Sand Castle. Kabbu is impressed by the structure of the castle, and wonders if their Queen would be jealous.
Inside the Sand Castle, the team flips switches which freeze all objects in a radius, similar to Leif's ice magic. The team finds Ancient Keys throughout the castle, which they use to unlock doors within it. They eventually find a Big Ancient Key, which unlocks a door on the top floor of the castle.
As they proceed through a hallway full of coffins with symbols of the Everlasting Sapling on them, the castle suddenly begins to shake. The team hears a voice shout in Roach dialect, which Leif translates to "This place is not for bugs. Leave or be BURIED!!!" The room begins to fill up with sand. A coffin opens, revealing a mummified roach, called the Watcher, who seems to wield the same magic as Leif, and attacks.
After being defeated by the team, the Watcher collapses, and disappears into the sand, leaving behind only a Peculiar Gem. Leif examines this gem, and concludes that it cannot be an artifact. Up ahead, the team finds the true final artifact, the Other Ancient Half.
- Astotheles (Mini-boss)
- Dune Scorpion (Mini-boss)
- Haunted Cloth
- The Watcher (Boss)
- Broodmother (Optional boss)
- Tidal Wyrm (Optional boss)
- Kali (Optional mini-boss)
- Cenn (Optional mini-boss)
- Pisci (Optional mini-boss)
- Vi's Requst
- It's Too Hot!
- Butler Missing!
- Power Plant Investigation.
- Bandit Hunt
- Stolen Item
- Explorer Check!
- Ore Wanted
- Lost Item
- This chapter has the most keys of any chapter in the game.