Kun
About
Nobody knows how many thousand li across it is. The back of it is a dark line on the horizon that sailors take for a coast and set courses by, and some of them make landfall on it, and the accounts of what that is like do not agree except that the ground breathed. It is not hunting. It has never noticed a ship in any account that survives. The danger is entirely that it is there, and that it moves, and that you were standing on it.
Stats
Save Rolls
Traits
Special Attack
It does not attack anything and has never been recorded doing so. Turns Over: no attack roll and no save. When it moves, everything on or near it moves with it. 300-ft radius, 3d10+6 bludgeoning to anything on the surface as the sea stands up, and vessels are simply gone. The DN should give a full round of warning - the horizon rising, the water going strange - because the only defence is not being there.
Special Defense
Terrain: it cannot meaningfully be damaged by a party. Treat the 300 HP as the reachable portion of an animal whose actual size has never been established, and reducing that to 0 achieves a wound it will not notice. Becomes the Peng: given time it transforms into a bird of the same impossible scale and leaves, and whatever it was lying in becomes somewhere else entirely.