In order to win the Lapis Lazuli for the Belt of Deltora, Adin was tasked by the leaders of the Mere tribe with killing a Sand Beast and bringing back its head. He encountered one early inside the Shifting Sands, but knew he had no chance of killing it. As the creature prepared to attack him, a lapis lazuli dragon appeared and killed it, allowing Adin to bring the head back to the Mere.
Secrets of Deltora
Doran the Dragonlover wrote of Terreocti in his book Secrets of Deltora, documenting details of its behaviour and sketching a diagram.
The Shifting Sands
Shortly after entering the Shifting Sands, Lief, Barda, and Jasmine discovered the dried remains of a stomach casing, though they did not know at the time what it belonged to. Later, a Terreocti emerged and killed the two Grey Guards who followed them into the sands. One of its stomachs broke off during the feeding and the companions watched the beast lay an egg inside it before moving along.
As the companions made their way towards the centre of the sands, a larger Terreocti tried to attack Lief, but Barda and Jasmine used blisters salvaged from the Grey Guards to keep it back. The blisters were unable to kill the sand beast, but it let them go when it realised they were heading to the Hive.
The Isle of Illusion
The Terreocti are mentioned as one of the monsters that Lief, Barda and Jasmine could not fight in their previous journeys.
The Sister of the South
Terreocti were mentioned by Lief when he thought over all the things the grey tide would destroy.
Terreocti are huge insect like creatures. When fully grown, Terreocti are as tall as three men. They have 8 limbs with claws on the bottom two and pincers on the other 6. Their heads are small and mostly taken up by their huge, reflective eyes.
All Terreocti have several sack-like stomachs on the front of their body. When a stomach becomes full, it tears away from the body and falls to the ground.
Terreocti are ambush hunters, lying in wait under the sands until potential prey comes walking by. To catch its prey, a Terreocti will hide beneath the sand and listens to vibrations in the sand caused by movement. Because of this, their eyesight is quite poor. A faint trickling of sand is usually all the warning they give before springing into an attack.
Female Terreocti will lay their eggs directly inside the fallen stomach, while males will beat the sand with their feet until a female arrives to lay an egg inside theirs. Afterwards, the Sand Beasts leave their young to grow alone, knowing it will have a good food supply when it hatches.
Their ambush hunting also keeps them safe from lapis lazuli dragons, who hunt them as a primary food source.The population of the Sand Beasts are directly linked to how much food is made available to them. In the ages before Deltora's unification, the Meresmen would dispose of prisoners taken in border wars by tossing them into the Shifting Sands. This influx of prey items saw a massive increase in the Terreocti population, even with Lapis Lazuli dragons preying on them. However, their numbers returned to a more stable level when Deltora was unified and the practice abolished. Even with the absence of Lapis Lazuli dragons, Sand Beast numbers never reached the same height as they did before Deltora's unification.
Terreocti are opportunist hunters who will eat anything they can catch, but their main source of food are Scorpion Lizards.
In the anime, adult Terreocti grow twice as large as their book counterparts. Their bodies are more segmented, and resemble a centipede and praying mantis hybrid. They also have only one eye.