A constant spell affects the monster without the monster needing to cast it, and its duration is unlimited. If a constant spell gets counteracted, the monster can reactivate it by spending the normal spellcasting actions the spell requires.
AC 48; Fort +42, Ref +34, Will +36
HP 430; Immunities bleed, paralyzed, poison, sleep; Resistances acid 20, bludgeoning 20, fire 20
Toxic Downpour(aura, primal)
Venomfist's presence alters the weather within 2 miles to heavy rainfall as long as they're located in a sizable body of water. When they become hostile, the rain in a 120-foot emanation from them becomes highly toxic and deals 8d8 poison damage (DC 42 basic Fortitude half) to all creatures in the area.
Vortex(aura, water)
120 feet. Water in the area that is in the same body of water as Venomfist is difficult terrain for Swimming creatures that don't have the water trait.
Speed 40 feet, swim 120 feet
Melee Fist +42 (magical, reach 20, unarmed), Damage 4d6+21 bludgeoning plus 4d6 poison plus Improved Grab
4d6 bludgeoning plus 4d6 poison, DC 45 basic Fortitude
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.
Create Poison
Frequency once per day
Effect Venomfist transforms a portion of their watery body into a single dose of any type of common poison of 20th level or lower—tears of death is their favorite. If they're holding a container, they can create the poison within that container, at which point the poison remains viable for 24 hours before it reverts to water. Alternatively, they can apply a contact or injury-created poison to their fist.
Surge
Venomfist expands their space to fill the area of their vortex. Creatures within the aura take 6d12+6 bludgeoning and 6d12+6 poison damage with a DC 45 basic Fortitude save. A creature that fails this save is pushed 40 feet. Venomfist then retracts to their former space and cannot use Surge again for 1d4 rounds.
Improved Grab
Trigger The monster's last action was a successful Strike that lists Improved 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 as a free action. This attempt neither applies nor counts toward the creature's multiple attack penalty.
The monster can instead use Improved Grab as an action and choose one creature it's grabbing or restraining with an appendage that has Improved Grab to automatically extend that condition to the end of the monster's next turn.