Initiating Vore: Whenever you take the Attack action on your turn, you can replace any or all of the attacks you make with Vore attempts. If you make a Vore attack, make an Athletics check, opposed by the opponent's Athletics or Acrobatics check. On a success, you begin to swallow the target, and for the rest of the turn you may not attack that same target. Roll a d10 and add your Strength modifier, and add a +2 bonus for every 5 your check exceeded the prey's by. Record this number; this becomes that prey's Swallow number, which represents how deeply they've vanished into their predator's body.
Continued Predation: Once you start swallowing a target, as an action you may continue to claim them. Make an opposed check as before; on a success, roll 1d10 plus your Strength modifier, with a +2 bonus for every 5 your check exceeded the prey's by, and add it to the prey's Swallow number. While swallowing a target, your speed is halved.
Claiming the Prey: If at any point the prey's Swallow number exceeds 15, the prey is completely trapped in their predator's body. While completely swallowed, the prey has disadvantage on escape attempts, and takes digestion damage at the start of each of their turns. While prey is completely inside, the predator can use an action to deal digestion damage to the target and increase their Swallow number by an amount equal to their Strength modifier. In addition, a predator is not encumbered by prey if the prey is completely inside.
Escaping: While a creature is being eaten, as an action the prey can attempt to escape their predator. The prey rolls an Acrobatics or Athletics check (their choice) opposed by the predator's Athletics check. As noted above, the prey has disadvantage if they have are completely inside of the predator. On a success, the prey rolls 1d10 and adds either their Strength or Dexterity modifier and subtracts this result from their Swallow number. If this decreaes their Swallow number to zero, they emerge, falling prone in an unoccupied space adjacent to the predator.
Swallow Cap: A prey's Swallow number cannot exceed 30, unless the predator has multiple stomachs. In that case, every 15 added to the Swallow number represents the prey being moved to a lower stomach, and the maximum value is instead 15 times one more than the number of stomachs the predator has. For example, if a centaur, which has two stomachs, swallows a target, the prey is moved to their hindbelly if their Swallow number reaches 30, and the Swallow number is capped at 45.
Digestion: Once prey begins to take digestion damage, the predator must decide whether they intend to digest or subdue the target. The type of damage dealt by the body depends on this choice: bludgeoning for subdual, and acid for digestion. The predator can change its mind at any time. If the prey is reduced to zero hit points by an organ that is subduing them, the prey stabilizes automatically.
Rescue: If the predator's target is not completely swallowed, an adjacent ally can take an action to free them. The ally makes an Athletics check, opposed by the predator's Athletics check. On a success, the ally rolls 1d8 and adds their Strength modifier, and reduces the prey's Swallow number by that amount. If this reduces the prey's number to zero, they are freed, but the ally falls prone from the effort. If a predator falls to zero hit points, any prey, whether partially or completely inside, can take an action to escape from the predator, emerging prone in an unoccupied space adjacent to the predator. An ally can also take an action to extract the prey. If the prey is in the second stomach (or deeper), they instead move to the previous stomach, and an ally cannot retrieve them.
Absconding: If a predator chooses to subdue a target, they wake with 1 hit point after 1d4+1 hours have passed, as normal. They do not take digestion damage unless the predator actively chooses to. Upon waking, the prey's Swallow number will be set to the maximum, unless the predator has multiple stomachs and chooses to store them in a specific one. (In this case, their Swallow number is that stomach's number plus 15.)