Graveshell

The graveshell conceals its true nature over the course of a few minutes by impaling a corpse (typically humanoid) on its spiky shell before partially submerging itself in water. It has an automatic result of 22 on Deception checks and DCs to pass as a corpse floating in the water.

This sense allows a monster to feel vibrations caused by movement through a liquid. It's an imprecise sense with a limited range (listed in the ability). Wavesense functions only if monster and the subject are in the same body of liquid, and only if the subject is moving through the liquid.

Trigger The graveshell takes damage from a physical attack while withdrawn into its shell Effect The graveshell reduces the damage from the attack by an amount equal to its shell's Hardness. A graveshell's shell has a Hardness of 6, 10 Hit Points, and a Broken Threshold of 5. If its shell is broken, the graveshell can't attack with shell spikes, nor can it use Shell Game. If the shell is destroyed, the graveshell's AC is reduced to 16. As long as a graveshell is alive, its shell naturally repairs itself after a week.

The graveshell can hold its breath for 200 rounds (20 minutes).

The graveshell withdraws into its shell, gaining a +2 circumstance bonus to AC and allowing it to use Shell Block. This lasts until the graveshell moves or attacks with any attack other than its shell spikes. The graveshell can't take this action or get any benefit from it if its shell is Broken.