How does forced movement affect a rider?

Question

The Player's Handbook specifies what happens if a rider's mount undergoes forced movement (p. 198):

If an effect moves your mount against its will while you're on it, you must succeed on a DC 10 Dexterity saving throw or fall off the mount, landing prone in a space within 5 feet of it.

What happens if the rider undergoes forced movement?

Typical causes of forced movement:

• Shoving melee attack
• Moved by a grappler
• Pushing Attack (Battle Master maneuver)
• Thunderwave spell (mount saves but rider fails)

Also, in which cases does a military saddle help? (Player's Handbook p.155):

A military saddle braces the rider, helping you keep your seat on an active mount in battle. It gives you advantage on any check you make to remain mounted.

• There's no check to be moved once grappled, only to be grappled in the first place. Does a military saddle give advantage to resist the initial grapple, or can a rider be pulled out of one just as readily as from a regular riding saddle?
• Thunderwave requires a single Dexterity save against both damage and forced movement. Does the spell save count as a check to remain mounted for the purpose of resisting damage? For the forced movement?

Answer 1

What Happens if the Rider undergoes forced movement?

PHB pg 198 also has a partially relevant sentence:

If you’re knocked prone while mounted, you must make the same saving throw.

While none of your above examples involve the 'Prone' status affliction, we can reason that under normal circumstances, if something were about to try and force move you (effectively off your mount), then you would be subject to the same saving throw to remain mounted as outlined above as you would if the mount were also being forcefully moved. (DC 10 Dex).

What about the military saddle?!

If you add in the military saddle, then you would have advantage on the DC 10 Dexterity saving throw to remain mounted.

This is an example of 'Specific beats general' rule of thumb. Were the general rule is found in spells like Thunderwave, that do not allow you save, they just work, and push you up to 15 feet; and the specific rule is what happens when you are mounted and you (or your mount) are forcefully moved while using a military saddle.

What about being grappled in your military saddle?

The grapple rules are on PHB pg 195:

"...you try to seize the target by making a grapple check, a Strength (Athletics) check contested by the target’s Strength (Athletics) or Dexterity (Acrobatics) check (the target chooses the ability to use). If you succeed, you subject the target to the grappled condition..."

Grappled: Your speed becomes 0...

Where as the other examples are doing something to impact you; a shove, a shockwave, a hard attack, the person grappling you is physically picking you up and moving you against your will (even though your speed is 0). So I would personally rule that you gain advantage on the check (because of the saddle) to avoid the grapple condition but once you fail and become grappled, you are at the mercy of your captor. You can continue to save and break free of the grapple, but you get no more advantage, and can do nothing to control being forcefully moved out of the saddle as you are grappled and no longer able to hold on to anything.

Application of the above example: Let's say a Kraken tried to grappled you (shame on you for riding your horse on the ship deck!), you could try and hold on to the saddle and fight away the tentacles, but if they overcame your resistance and wrapped themselves around you; you would be effectively helpless to do anything that would gain you advantage once it tried to pull you out of the saddle and see how tasty you are. You struggle as you are lifted out of your mount and over the water, drawing closer to the creatures maw, but you break free on your own just before you become Kraken food.

Answer 2

You have multiple questions in your question.

1. The Thunderwave save is, as you note, for both reducing damage and overcoming the "push" from the Thunderwave. By RAW, if the spell is saved against there is no need to save again. You have already saved against the push.

   PHB p. 283, Thunderwave spell description, is quite clear on this matter.

   On a successful save, the creature takes half damage and isn't pushed.

2. If someone attempts to grapple you while in the saddle, the initial advantage does not carry over once you fail the grapple resistance. At that point, you are trying to break the grapple and use the rules for that. There is no language that indicates that the saddle gives you advantage in grapple breaking.

   (PHB. p.155):

   A military saddle braces the rider, helping you keep your seat on an active mount in battle. It gives you advantage on any check you make to remain mounted.

   That would include someone attempting to use a grapple to dismount you.

Answer 3

To restate the question:

What happens when a mounted rider is involuntarily moved?

I don't feel this question has been answered. Possibly because the example spell allows a save. Possibly because the seemingly unrelated prone condition is discussed. Possibly because there is no RAW answer, but there is an answer to the military saddle question so people busy themselves with answering the questions they can.

To make the OP question a bit clearer consider the single case of a rider hit with a repelling blast. Something is moved. That something is either the rider or the rider and the mount. The only exception I can think of would be a rider directly in contact with a wall (which I'm having some trouble envisioning if the rider is astride a mount). Even then an argument could be made that you were involuntarily moved without leaving your square and therefore potentially unmounted.

The answer must be:

1. The rider is moved and unmounted,
2. The rider and the mount are treated as a unit and you are both are moved but you remain mounted,
3. The rider and the mount are a unit so they both move, then make a save to see if you are unmounted.

At least that's all the answers I can think. I exclude the rider grapples the mount as breaking too many other grappling rules and in any case would be the same answer as 1. I excluded you don't fall because that doesn't answer the question of what is moved.