What would a mounted creature, or one steering a vehicle, do if affected by the harpy’s Luring Song?
Would the creature dismount/leave the vehicle and proceed to the harpy under their own power? Or would they drive their mount or steer their ship toward the harpy?
Things to bear in mind:
The paragraph under the “Harpy Song” heading on MM p.181 informs us that the whole point of Luring Song is to cause targets to “blunder” into hazards like into quicksand and over cliffs. Obviously, the target should be incapable of making an analysis as to which route is safest, fastest, etc. Otherwise the charm is trivialized to insignificance. If speed was their primary concern, the target could say “oh those razorvines are going to slow me down; better find another way round” so it’s safe to say that the intent of the “most direct route” wording of the Luring Song ability means they will choose a path which is the closest to a straight line as possible. That straight line might take them into a choppy sea if they are standing on a sea cliff, but if they are on a boat in the same choppy sea, do they jump in and risk drowning, or ram their boat aground? I’m looking for a way to adjucate scenarios like this consistently.
A creature affected by Luring Song is incapacitated and therefore cannot take actions or reactions. In fact all they can do is move toward the harpy. Can an incapacitated creature even still control a vehicle or a mount? Back to the Luring Song, before it was changed by errata, the MM used to say the target can take the dash action. So Dash let’s you move up to your speed. Vehicles and mounts have speeds, but the don’t give you a speed so there is no dashing in a vehicle or on a horse (a horse can take the dash action but that’s not the same thing in this case). To dash you’ve got to use your legs, yet moving overwise was optional. It may be helpful to consider the intent of the change. Obviously getting closer to the harpy is non-optional now (even though it still never says you must use your FULL movement speed). But did they mean to allow targets to use other means of moving? Does saying the target must “move” refer to the Movement and Position rules?