Depends what you mean by engaged? A standard weapon attack should only be hitting and damaging one creature at a time, but if you have extra attack and then you could make one attack on each.
There may be some monster stat blocks that combine rider and creature into one, I can't think/remember any examples but I imagine it exists, or some DMs do it for their homemade monsters, to simplify combat slightly.
If you are attacking with a melee weapon (lets say it’s not a reach weapon) you are adjacent to both mount and rider. You can attack either, but would need to do it separately for each creature (as swiftsign said) damaging one would do no damage to the other.
If you moved away, both of them would be able to use their reaction to attack you. If they moved away, you would be able to use your reaction to attack one or the other, but not both. (Though this gets a little weird, because it’s the rider’s move action, but the mount is the one using their movement, so your DM may only allow you to attack one or the other, instead of giving a choice)
Them attacking you gets a little wonky with the mounted combat rules bumping up against a dragon being intelligent, so it’s going to depend on the DM is running them to tell if only the rider can attack you, or both rider and mount. Since if the rider is controlling a mount, it can’t take the attack action, but the mount, acting on its own, might choose to take the attack action.
The only way in which mounted combat of this sort will violate the standard RAW - dragons are explicitly listed as acting under the independent mount rules - is that the AoO rules are modified such that if the dragon provokes an AoO, you can take it on the rider (if the rider is within reach at the time of the AoO, of course - the rules don't make your weapon longer). Distances are measured independently (in both directions - the rider doesn't get to do anything bizarre like measure a bowshot from the dragon's body), so you may develop a rules headache if you're playing on a grid - it's quite easy to have a dragon mount such that if the dragon is fitted to the grid, the rider isn't properly in a grid spot. That's not even specific to dragon mounts or anything - most mounts aren't 3x3 grid squares (i.e. standard size for Huge) with a saddle perfectly in the center grid space.
Also bear in mind that if the mount is large enough, it may enable riding solutions superior to a saddle. The Ogre Howdah in Mordenkainen's is a great example of this. That can change the mechanics of combat as well.
If I make a melee attack against a human enemy who is mounted on a dragon, am I then engaged with the human, or both the human and the dragon?
If I'm engaged with both, then does the attack roll or attack damage, also affect the dragon?
Thanks!
Depends what you mean by engaged? A standard weapon attack should only be hitting and damaging one creature at a time, but if you have extra attack and then you could make one attack on each.
There may be some monster stat blocks that combine rider and creature into one, I can't think/remember any examples but I imagine it exists, or some DMs do it for their homemade monsters, to simplify combat slightly.
“Engaged with” isn’t a term in D&D.
If you are attacking with a melee weapon (lets say it’s not a reach weapon) you are adjacent to both mount and rider. You can attack either, but would need to do it separately for each creature (as swiftsign said) damaging one would do no damage to the other.
If you moved away, both of them would be able to use their reaction to attack you. If they moved away, you would be able to use your reaction to attack one or the other, but not both. (Though this gets a little weird, because it’s the rider’s move action, but the mount is the one using their movement, so your DM may only allow you to attack one or the other, instead of giving a choice)
Them attacking you gets a little wonky with the mounted combat rules bumping up against a dragon being intelligent, so it’s going to depend on the DM is running them to tell if only the rider can attack you, or both rider and mount. Since if the rider is controlling a mount, it can’t take the attack action, but the mount, acting on its own, might choose to take the attack action.
The only way in which mounted combat of this sort will violate the standard RAW - dragons are explicitly listed as acting under the independent mount rules - is that the AoO rules are modified such that if the dragon provokes an AoO, you can take it on the rider (if the rider is within reach at the time of the AoO, of course - the rules don't make your weapon longer). Distances are measured independently (in both directions - the rider doesn't get to do anything bizarre like measure a bowshot from the dragon's body), so you may develop a rules headache if you're playing on a grid - it's quite easy to have a dragon mount such that if the dragon is fitted to the grid, the rider isn't properly in a grid spot. That's not even specific to dragon mounts or anything - most mounts aren't 3x3 grid squares (i.e. standard size for Huge) with a saddle perfectly in the center grid space.
Also bear in mind that if the mount is large enough, it may enable riding solutions superior to a saddle. The Ogre Howdah in Mordenkainen's is a great example of this. That can change the mechanics of combat as well.
Hi, welcome to D&D where only the target of an attack takes damage.