A thunderclast is an "enormous stone beast" from the times of the Desolations, and does not normally exist outside Desolations.
In the Prelude to The Stormlight Archive, one of these beasts is described as "vaguely skeletal in shape, with unnaturally long limbs that sprouted from granite shoulders. The eyes were deep red spots on the arrowhead face." It had "riblike protrusions from its chest" and its "hand was as long as a man was tall."
During one of Dalinar's visions at the Purelake, the body of the creature he saw was "like a full figure, easily six feet. A swimming person, but like a shadow." Further, it was described as "skeletal, with thin bony limbs and spiky fingers that ended in rocky claws." Its chest was a rib cage of stone and it stood thirty feet tall.
Thirty-one years ago from the present day, while out in a highstorm, Dalinar saw a luminous, gargantuan figure that moved on spindly glowing legs. Dalinar gave the whatever-it-was a rude gesture, then moved indoors.
During the siege of Kholinar, the creature that crests the wall is described as having an enormous stone wedge of a face that reminded Adolin of that of some greatshell beast, though its eyes were just red spots glowing from deep within.
They are capable of ripping guard towers apart, and casually flipping boulders the size of a horse.
Venli witnessed as thunderclasts awakened:
Among the waiting spirits were two larger masses of energy - souls so warped, so mangled, they didn't seem singer at all. One crawled into the stone ground, somehow inhabiting it like a spren taking residence in a gemheart. The stone became its form.
Then it ripped itself free of the rock. Around her, the parshmen stumbled back in awe, so surprised that they actually drew spren. The thing loomed over the human forces, while its companion climbed into the stone ground, but didn't rip out immediately.
There was one other, mightier than even these. It was out in the water of the bay, but when she looked into the other world, she couldn't help but glance toward it. If those two lesser souls had had created such dauntless stone monsters, then what was that mountain of power?
Speaking to his newly herself-named Shardblade, Adolin addresses Mayalaran and says, "Let's bring that thing down." He was barely as tall as the thing's calf when he attacked it, and didn't use any of the sword stances with which he was familiar; he just hacked at it as if he was attacking a wall, slicing right along the top of the thing's ankle.