Tremorsense allows a monster to feel the vibrations through a solid surface caused by movement. It is an imprecise sense with a limited range (listed in the ability). Tremorsense functions only if the monster is on the same surface as the subject, and only if the subject is moving along (or burrowing through) the surface.
Slow
The zombie chuul is permanently Slowed 1 and can't use reactions.
AC 22; Fort +18, Ref +10, Will +13
HP 200, void healing; Immunities bleed, death effects, disease, paralyzed, poison, unconscious; Weaknesses vitality 15, slashing 15
Void Healing
A creature with void healing draws health from void energy rather than vitality energy. It is damaged by vitality damage and is not healed by vitality healing effects. It does not take void damage, and it is healed by void effects that heal undead.
1d12+7 bludgeoning damage, DC 25 basic Fortitude (Grabbed by claw only)
The monster deals the listed amount of damage to any number of creatures Grabbed or Restrained by it. Each of those creatures can attempt a basic Fortitude save with the listed DC.
Mandibles
Requirements A creature is Grabbed and Paralyzed by the zombie chuul's tentacles
Requirements The zombie chuul has a creature Grabbed
Effect The zombie chuul transfers the grabbed creature from its claws to its tentacles, or vice versa. The creature is exposed to the zombie chuul's paralytic venom when transferred into the tentacles and at the start of each of the zombie chuul's turns if it remains grabbed by the tentacles.
Grab
Requirements The monster's last action was a successful Strike that lists Grab in its damage entry, or the monster has a creature Grabbed or Restrained
Effect If used after a Strike, the monster attempts to Grapple the creature using the body part it attacked with. This attempt neither applies nor counts toward the creature's multiple attack penalty.
The monster can instead use Grab and choose one creature it's grabbing or restraining with an appendage that has Grab to automatically extend that condition to the end of the monster's next turn.