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I have a wizard casting Lightning bolt at a flying mount (Griffon), hoping to hit both the mount and its rider. The DM has determined that I have hit the mount, but we have the question of if this also hits the rider as well?
How does Lightning Bolt work when targeting both a mount and the rider along the line effect area?
One important question is whether a mount and its rider are considered to be occupying the same space, or whether the rider occupies a space above the mount's space. If they occupy the same space, then any area of effect that hits the mount can also be aimed in such a way that it hits the rider (although it is not guaranteed, since the mount is one size class larger, so it may occupy more space than its rider). If a rider is considered to be occupying a space above their mount, Then a horizontal area of effect, such as a Lightning Bolt fired approximately horizontally, will hit either one or the other but not both. That is, you could aim exactly at ground level and hit the mount, or aim at a slight upward angle and hit the rider on top of the mount.
Unfortunately, I can't find anything in the rules that specifies which of the above is the correct interpretation. The rules explain how to mount or dismount a creature, but they don't specify what that means in terms of relative positioning. This may be because there are a wide variety of possible shapes and sizes of mount (and rider!), so a general rule for all mounts might not make sense. Hence, the DM will need to make a ruling for a particular case.
Personally, I would rule that in the case of a griffon and humanoid rider, there is more than enough overlap between a rider and mount that there would be no problem hitting both with the same Lightning Bolt spell, regardless of the angle it is fired from.
Since you mentioned that the mount in question was flying, and presumably the wizard casting Lightning Bolt was on the ground, it is likely that the spell was fired at a potentially steep upward angle. In this case, even if the DM ruled that the rider occupied a space above the mount, the spell could still easily hit both of them if the angle is steep enough. In the most extreme case of firing 90 degrees straight up, the spell almost certainly will hit both of them.
The other possibility that might result in the rider not being hit by the spell is if the spell were entirely blocked by the mount, thus protecting the rider. However, a creature generally does not provide sufficient cover to block a spell. Generally, creatures provide at most half cover:
A target has half cover if an obstacle blocks at least half of its body. The obstacle might be a low wall, a large piece of furniture, a narrow tree trunk, or a creature, whether that creature is an enemy or a friend.
When shooting at a rider on top of a mount that is flying above you, it would be reasonable to grant the rider half cover (and to apply the optional rule about hitting the cover on a miss). However, as described in the section on areas of effect, only total cover is sufficient to block a spell's effect:
A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover.
Hence, as long as the Lightning Bolt passes through both the mount's and the rider's spaces, it will hit both of them, and neither one will block the spell from hitting the other.
It should hit both the mount and rider, given that is is cast at the right place. Lightning bolt targets a place. Anyone in the way is hit. On a grid you aim at a square.
It's 5 feet wide. While not purposefully built for attacks into the air, I would also rule it is 5 deep high as well. I can't find documentation to that effect, but it isn't a sheet of lightning, and you don't hit differently against halflings and giants. So let's not complicate the system by creating new complex rules on what exactly is in the path of the spell.
There is a Mike Mearls tweet that agrees that a lightning bolt affects any creature it partially hits.
https://twitter.com/mikemearls/status/749701115923484672
Mount and rider would of course have separate saves.
Both the rider and the mount should make dexterity saves.
Lightning bolt reads:
A stroke of lightning forming a line of 100 feet long and 5 feet wide blasts out from you in a direction you choose. Each creature in the line must make a Dexterity saving throw. A creature takes 8d6 lightning damage on a failed save, or half as much damage on a successful one.
A rider usually sits directly on their mount or on a saddle. This means that rider and mount are no more than a few inches away from each other. The player could then chose the direction of the bolt to be where the rider's body meets the mount's. (Lightning bolt just hits, otherwise this would be a targeted strike.) At 5 feet wide (whether that means a plane or a circular beam), the bolt can hit the rider and the mount, 2.5 feet for the rider, 2.5 for the mount.
Since lightning bolt extents 100 feet, which usually is played as going through enemies, it's reasonable to assume this attack works even if you can't see the intersection of bodies, as might be the case when the pair are flying. I'm not sure how it would work if you can't see the rider.
A stroke of lightning forming a line of 100 feet long and 5 feet wide blasts out from you in a direction you choose. Each creature in the line must make a Dexterity saving throw. A creature takes 8d6 lightning damage on a failed save, or half as much damage on a successful one.
A very strict reading of this suggests only the mount would be hit as the beam has a length and a width but no height is mentioned. Which is why it is described as a line.
This means that if you aimed at the body of a Gryphon you would miss the rider so, in very pedantic terms, the DM is correct. In this strict interpretation you would have to be directly under the Gryphon, or at a similar steep firing angle, to catch them both.
But that isn't a lot of fun is it? You could argue that mounting a Gryphon would be similar to mounting a horse and so the rider's leg is covering part of the body of the Gryphon and therefore would get hit but it isn't a really compelling argument.
The DM could also beware of creating situations in the future where they would have to abide by the same rules to be consistent and so is protecting themselves from that (I have a canny group that come up with scenarios based on house rules/discussions we've had before so I know I need to be careful about what I say now because it WILL come back to bite me).
EDIT
The section of the PHB that refers to areas of effect mentions that a line:
A line extends from its point of Origin in a straight path up to its length and covers an area defined by its width.
I don't know what the writers meant by that but it could potentially give some leeway for arguing that they only gave one dimension but meant for it to define two. Personally, it's not something I'd support though.
RAW may not be 100% clear, but logically it would seem that both would be affected. As others have stated each creature in the 100ft "line" would be subject to the Lightning Bolt. Cover may be an issue? Electrically conductive armour or barding may also be considered by some DMs. I would just rule that both are included, unless some specific ruling overrides that situation.
