Intercepting a grappled Bite Swallow attack in place of an ally with Vigilant Guardian: What happens?

Question

Let's use a Giant Toad as an example:

Bite. The target is grappled, escape dc 13 Until this grapple ends, the target is restrained, and the toad can't bite another target

Swallow. The toad makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends. The swallowed target is blinded and restrained, it has total cover against attacks and other effects outside the toad, and it takes 10 (3d6) acid damage at the start of each of the toad's turns. The toad can have only one target swallowed at a time.

Let's also say someone in the party is a Mark of the Sentinel Human:

Vigilant Guardian. When a creature you can see within 5 feet of you is hit by an attack roll, you can use your reaction to swap places with that creature, and you are hit by the attack instead. Once you use this trait, you can’t do so again until you finish a long rest.

If the Mark of Sentinel Human uses Vigilant Guardian to intercept an attack on an ally who's being grappled and bitten by a Giant Toad, what happens?

1. Is anyone swallowed, and if so who?
2. Does the Vigilant Guardian user become grappled?
3. Does the restriction of Bite grapple/Swallow only able to be used on one target at a time come into play?

Answer 1

The Toad decides what happens

Normally an attack has three stages: Choose a target, determine modifiers, and resolve the attack. The third, 'resolve the attack' stage, says:

You make the attack roll. On a hit, you roll damage, unless the particular attack has rules that specify otherwise. Some attacks cause special effects in addition to or instead of damage.

While vigilant guardian says:

Once a creature that you can see within 5ft of you is hit with an attack roll, you can use your reaction to swap places with the creature, and you are hit by the attack instead.

The vigilant guardian complicates the attack by interrupting the third stage. The attack roll is made, the DM determines that the attack was a hit, but before the damage and special effects can be applied, vigilant guardian is invoked to switch the target of the attack. However, this feature says that the guardian is still hit by the attack. They would thus suffer the normal damage and special effects of the attack.

Suppose a Giant Toad begins its turn with your ally already grappled because of a Bite on its previous turn. It elects to use its Swallow Action, which says:

The toad makes one bite attack against a Medium or smaller target it is grappling. If the attack hits, the target is swallowed, and the grapple ends.

Because it is using its swallow action, the Toad makes a Bite attack against your grappled ally, and hits. You invoke vigilant guardian, such that the Bite instead hits you. However, this runs afoul of the description of the Toad's Bite, which says that on a hit:

the target is grappled (escape DC 13). Until this grapple ends, the target is restrained, and the toad can't bite another target.

Thus we have a conundrum. Your vigilant guardian just made you the target of a successful Bite attack by the Toad, but the Toad cannot Bite you because it is currently grappling your ally.

If the Toad does nothing else, its attack has been spoiled, because you have made the target of its attack invalid. Your ally is still grappled, but the Toad did not make a successful Bite attack, and neither one of you takes damage or is swallowed.

However, the Toad can choose to drop its grapple on your ally. The grappling rules say:

If you succeed, you subject the target to the grappled condition. The condition specifies the things that end it, and you can release the target whenever you like (no action required).

There are no timing rules for when something that happens 'whenever you like' is resolved, but if we consider your use of vigilant defender and the Toad's ending of grapple to be simultaneous effects, then the optional timing rules in XGtE suggest that on the Toad's turn, it would be able drop its grapple in response to your ability and still have its response resolve first. Thus by the time you had made yourself the target of its Bite attack, you would be a valid target because it was no longer grappling your ally.

In that case, you would be hit by the bite, take damage, and suffer the special effects of being grappled and restrained, while the Toad would then be unable to grapple another target.

Once the Bite had resolved, then the Toad's swallow would continue:

The toad makes one bite attack against a Medium or smaller target it is grappling.
[At the time the Toad made the Bite attack, the attack was made against your ally, who at that point was grappled, so the swallow and Bite were both legitimately initiated]
If the attack hits, the target is swallowed, and the grapple ends.
[The attack hit, at which point you made yourself the target, and it says that the target is swallowed.]

Thus, as a result of choosing to become the target of the Toad's Bite attack, you would be swallowed.

In short, if the Toad maintains the grapple on your ally, they are still grappled but neither of you are hit with the Bite. If the toad chooses to drop your ally in response to your vigilance, your ally is no longer grappled, but you are Bitten and swallowed.

Narratively, in the first case the Toad is about to swallow your ally, but you present yourself so forcefully that it hesitates and its attack is lost. In the second case, just as your ally is about to be swallowed you move in front of the Toad and it drops your ally and swallows you instead.

Answer 2

Contradictions are why DMs exist

I think you've pretty much identified the problem yourself. The rules are contradictory:

1. The Giant Toad cannot attack any other target than the grappled
2. You can force it to attack you
3. You are not grappled

For another example: What if instead, the creature being attacked was at the other side of a wire fence? Technically it is "a creature you can see within 5 feet of you", but it wouldn't make sense for you to be able to swap places with it. RAW I suppose you would teleport through the fence.

I don't think there is a good RAW interpretation here. I suspect no matter what there will be corner cases that breaks the argument or immersion. That's OK! DnD was designed with the DM in mind.

From the introduction of the Dungeon Master's Guide, in the section on The Dungeon Master.

The D&D rules help you and the other players have a good time, but the rules aren’t in charge. You’re the DM, and you are in charge of the game.

Let's leave RAW for a moment: What could make sense? Well, all of your examples could. I read Vigilant Guardian as the intention is the Sentinel Human jumping in front of danger, shielding the target with their body. If so it makes sense they should suffer the consequences of being hit.

Does that mean they should get swallowed? I'd say yes. Would it be OK, if they just hit by the Bite instead? Sure. Would it be game breaking if we ruled the toad got confused and didn't attack at all? No problem.

This is a feature that only works once on a long rest. So as a DM: Feel free to tell your best story. It won't break your game.

Answer 3

Multiple things could happen, depending on the positions on the battlefield.

Relocation effects are interesting, especially when you combine them with things like grappling rules and things that are effectively instant teleportation. Let's start off with a clarifying statement about one of the ways to end the grappled condition.

The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.

To trigger Vigilant Guardian, you must be within 5 feet of the attacked target. This means if you are in a square five feet from the target and the Giant Toad, when you "swap places" with the attacked target, RAW, you and the target switch spaces. While narratively I agree it would make sense for you to essentially pull a Naruto-esque replacement and swap places inside the grapple via a teleport effect, relocation effects alter locations, not conditions, and grappled is a condition. In this case;

1: No one is swallowed, because the Vigilant Guardian hit with a bite attack (the specific method called out to make a swallow) is not the one being grappled by the toad.

2: The original target of the first bite attack was not transported out of the Toad's reach; it is still grappled. The Vigilant Guardian may be grappled following being hit by a bite attack, as per the description of the bite attack, if the toad releases the original target (the replacement effect did not move it out of reach to break the grapple).

3: The restriction on targeting has already taken place, and was followed appropriately, resulting in a hit. We are replacing the taker of a hit with a feature, similar to Arcane Ward, not reselecting targets, like Sanctuary. The Vigilant Guardian takes damage, but is not swallowed.

If, however, you are within five feet of the target, and ten feet away from the Giant Toad when you activate Vigilant Guardian.

1: No one is swallowed, because the target struck (Vigilant Guardian) is not grappled when struck by the bite attack.

2: The Vigilant Guardian does become grappled when struck, as per the description of the Giant Toad's bite attack. The original target is now 10 feet away from the toad, and this breaks the original grapple.

3: No, for the same reasons in the first setup.