For such a relatively new monster (first appearing in the 3.0 Monster Manual), the chuul has managed to worm (slither? crabwalk?) its way into nearly every campaign that touches water. In Pathfinder, the chuul’s CR of 7 makes it a lethal challenge at low levels and a fierce side obstacle or villain’s lackey (likely magically coerced) in mid-level adventures. And there is no moral quandary about slaying chuuls—while they are as smart as humans, they are uncommunicative, totally evil, and devoted to their horrific trophy collections.
A band of lizardfolk warriors comes out of the swamps at dawn. They surround the local chapel of the Copper Chalice during services, then kneel and offer their spears. Three months ago they killed and devoured a young traveling priest of the same faith, and then were immediately set upon by a chuul. Now they’ve lost a quarter of the tribe to chuul predation, and are convinced that by converting to the Chalice they will be spared.
An extraordinary tribe of troglodytes—led be a particularly clever cave druid with white dragon blood—have taken to ambushing their enemies near a chuul lair, safe in the knowledge that the aberrations will attack their foes and leave them unmolested.
A chuul is spotted at night carving strange runes with its claws on the sides of the barges lined along the quay. A witch on a broom of flying, part of the local messenger service, arrives with news that the same is happening up and down the delta. Further scrying reveals that the chuuls are all acting in concert, as if directed from afar by some sinister marionette.
—Pathfinder Bestiary 46