Added to
Homebrews.
Here's how we handled the venus fly-trap's grabbing n' drowning mechanic:
Jaws (Ex): Each of a giant venus fly-trap's jaws may make a single bite attack each round. It a jaw hits with its bite attack, the fly-trap can then attempt to start a grapple as a free action without provoking an attack of opportunity. If it wins the grapple check, it establishes a hold and can attempt to swallow the foe the following round. A jaw enganged in a grapple cannot make a bite attack, but the giant venus fly-trap is not considered grappled.
With each of its jaws, a giant venus fly-trap can try to swallow a single grabbed opponent of a smaller size than itself by making a successful grapple check. Once inside, the opponent takes 1d6+5 points of crushing damage plus 5 points of acid damage per round from the plant's fluids. A swallowed creature can cut its way out by using a light slashing or piercing weapon to deal 10 points of damage to the jaws (AC 12), destroying that set of jaws (and dealing 5 points of damage to the fly-trap) in the process.
An opponent not currently swallowed can attack a giant venus fly-trap's jaws with a sunder attempt as if they were weapons, but risks injuring the trapped victim (a 50% chance). A giant venus fly-trap's jaws have 10 hit points each. If a giant venus fly-trap is currently grappling a target with the jaw that is being attacked, it usually uses another jaw to make its attack of opportunity against the opponent making the sunder attempt. Severing one of a giant venus fly-trap's jaws deals 5 points of damage to the creature. The creature regrows severed or destroyed jaws in 1 week. A non-swallowed creature may also attempt to free the victim by grappling the fly-trap and succeeding on a second grapple check to open the jaws.
Here's another somewhat similar ability...
Consume (Ex): As a free action, a black willow can create or close a 5-foot-square opening in its body. It can dump an adjacent flat-footed or helpless Medium or smaller creature into such an opening and tilt its limbs as necessary to slide the creature down the black willow’s hollow interior. A conscious creature is entitled to a single DC 23 Reflex save to grab the sides and climb out. In that same round on the black willow’s action, digestive sap rises from the roots and begins to fill the interior. All creatures inside take 1d4 points of acid damage. As the sap continues to fill the chamber, the acid damage increases by 1d4 each round (2d4 points of acid damage in the second round, 3d4 points in the third, and so on) up to a maximum of 10d4 acid damage per round. The acid completely destroys any creature that reaches -10 hit points, preventing any form of raising or resurrection that requires part of the corpse.
A creature in the interior can cut its way out by using a light slashing or piercing weapon to deal 20 points of damage to the black willow’s interior (AC 15). Once the creature exits, the hole closes; another opponent inside must cut its own way out. Once all creatures inside the willow have escaped or been destroyed, the acid drains back into the roots at the same rate it filled the trunk cavity. A Huge black willow's interior can hold 4 Medium, 16 Small, 64 Tiny, or 256 Diminutive or smaller creatures. The save DC is Strength-based.