Eternity

The Leviathan is a beast which has never been seen in life, and is known solely to exist, or have existed, at all only by a handful of fossil and sub-fossil remains which have slowly been unearthed by erosion over many thousands of years far underground in the upper cave systems of the Wumbian basin. A true behemoth, the subterranean crustacean's limbs alone, turned to tall, eternally extended spires of stone,  may stretch sixty feet from its carapace. Given the locations where the remains have been discovered, the enormous animals are presumed to exist as large ambush predators, burying themselves when young below open 'skylights' in the caves in a grave they will spend their entire lives in, killing and consuming all that stumbles in upon them with vicious pincers and an inescapable vice grip. Given the scarcity of remains, it is likely that the Leviathan - if it still exists today at all - exists in incredibly low population densities, making their hidden, grim living as giant alien antlions until eternity wins out over them and their shells solidify to solid rock, until the workings of time eventually unearth their long-expired stone-cast bodies into the light of day once again. Nothing is known of how they breed, how long they live, or how the animals look nor behave, or would have behaved, as juveniles - and the entire adult animal, the posterior regions in particular which typically remain buried in stone, itself has never been fully observed.