The Jade Sentinels are an ancient evil, that are trapped within The Sands. The Sands were created to help trap the Sentinels. Israphel and his cult seem to be attempting to reactivate the Sentinels as part of their evil plot to conquer the world.
The giant hand of one Sentinel was discovered by a team comprised of Templar Enoch, Professor Grizwold, Templar Kesha and Scribe Anno. However, there were no survivors other than Grizwold (who left before venturing deep into the sentinel), as the corruptive influence of The Sands slowly drove Templar Enoch insane and he killed off the others before killing himself because of his rampant insanity.
The mysterious builders of the Jade Sentinels seem to have built a power station to activate them in an end-of-the-world scenario. King Finbar activated the system, though he was killed in the resulting fight with the three heroes. The system was ready to begin three days and seven hours after Finbar's death.
Later, the three heroes Xephos, Honeydew and Knight Peculier discovered "the hand" as well as the diary and the mutilated remains of the insane Templar Enoch. Later, while exploring the insides of the Sentinel on minecarts, Knight Peculier tragically died when his minecart fell through a break in the tracks and into the radioactive blood of the Sentinels, (although it's widely believed by fans that he survived the accident). Xephos and Honeydew were left alone to search the Sentinel without their trusted ally and old friend. Within some of the Sentinels, Israphel had placed cloning facilities, which Israphel had used to clone an army of minions to fight for him.
Later, Professor Webley, whom the heroes saved from the cloning vats, told the heroes that if the Jade Sentinels ever awaken, Minecraftia would surely be doomed. Which means that the Sentinels are probably even more powerful than Israphel himself. The heroes didn't seem to notice a sign onboard the Sentinel that stated there is a "dragon finder" within the Sentinels. This could mean Israphel intends to "awaken" the sentinels to locate the remaining dragons of Minecraftia (e.g. The Stone Dragon near The Tomb of Lehparsi) as a part of his plan.
The only known Sentinel has the build of a massive robot and many mechanical parts, including a giant hand and head which stick up through the Sands and into the air, a radioactive liquid for blood, a cloning machine which produces defective duplicates of the person cloned and a separate power core. It was only to be activated in an end-of-the-world scenario and the Sands were built to imprison it despite their own incredibly dangerous power, suggesting that the power of the Sentinel is even greater.