Can Mounted Combatant still force a Swallowed enemy to attack the rider?

Question

I'm looking at making a Small-sized Ranger character that has a Giant Frog for its animal companion and wondering if the following scenario is possible:

Round 1: Ranger is unmounted (is that necessary with Extra Attack?) and directs Giant Frog to grapple a Small enemy.

Round 2: Ranger uses Action to direct Giant Frog to Swallow the enemy, then mounts the Giant Frog.

All following rounds: The enemy attacks and Ranger uses Mounted Combatant to force it on himself (with disadvantage from being restrained and is effectively stuck inside the Giant Frog).

Thus, unless the Giant Frog dies to an AoE or some other residual effect, the enemy is essentially stuck inside the Giant Frog until it dies, yes?

Answer 1

Mounted Combatant does not work in this case

This is a very clever idea. Unfortunately, the negative synergy of the cover rules and of the Mounted Combatant feat prevents it from being useful.

The typical case where Mounted Combatant is useful is when you are mounted on a steed, and people try to cut down the steed to cut down your mobility. By forcing the attacks to be made against you, you are protecting your steed from harm.

However, when you are behind total cover from the enemy, the rules for total cover come into play:

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

So, we come into a contradiction between two rules: Mounted Combatant allows you to force the attack on you instead of your mount, but total cover disallows the attack on you entirely. Which one wins?

The answer is Specific vs General. While you can generally force the attacker to target you instead of your mount, this does not work when the attacker specifically has total cover against you. This is because their total cover prevents them from attacking you at all.

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Bonus: Ways they can escape, aside from killing the Giant Frog

There are other ways small creatures can escape, aside from trying to kill the Giant Frog. Here is an incomplete list, which may help you anticipate some of these when you use this build idea in your game.

• Find Familiar's vision swap (Action) + Misty Step (Bonus Action)

• Clairvoyance (Round 1 Action) + Misty Step (Round 2 Bonus Action)

• Enlarge/Reduce or Wildshape: The small creature can Enlarge itself or Reduce the Giant Frog. Either way, the Frog may have to spit out the swallowed creature. This is not necessarily what happens, depending on the DM. But as the Giant Frog can only swallow small creatures in the first place, it makes sense that if that swallowed target grew in size while swallowed, the Frog has to spit it out.

• High level escape spells: Freedom of Movement, Dimension Door, Teleportation, Plane Shift, Etherealness

• Gaseous Form: the creature can pass through pin-hole sized openings while in this form.

• Stinking Cloud: on a failed Con save, a target inside the area spends its turn "retching and reeling" -- as in, vomiting.

• Pushing Attack: On a hit, the Giant Frog must succeed a Strength save or be pushed back 15 ft away from you. The exact thing that happens here is DM-dependent, though, as it's easy to imagine a DM who will disallow this on the basis of reality (you can't push someone away from you if you are inside them, after all).

Answer 2

The following things are in conflict:

• The Mounted Combatant rules are simple and have no constraints on whether you can re-direct an attack, other than being mounted on the steed at the time.

• The rules for being swallowed are also relatively absolute about having no interaction with effects outside of the swallowing creature.

It is not made clear whether the re-direction of the attack is subject to the constraints of interacting with a swallowed creature. That is not too unusual, the possible number of special case combinations is huge, and 5E avoids handling this sort of thing with deeper abstract rules (e.g. by defining terms such as line of sight, line of effect, labelling all abilities according to whether they are constrained by them etc).

This will come down to DM ruling. The mechanical rules in 5E are not made to be robust against edge cases constructed like this, and there is no expectation of rulings on the side of gamist vs simulationist in general (or "play how you like").

As a personal opinion, I would rule against use of re-directing the attack in this scenario, as it would seem to lack any game world mechanism. There is an implied mechanism in the feat of being a skilled and careful rider, able to anticipate targets of the attacks and change position or interact with the incoming attack - nothing like this mechanism could be possible against an attack that originated inside the mount.

Potentially spells that re-directed effects of attacks would work e.g. the Cleric spell Warding Bond, but these have other limitations.