Do you remember what the automaton was called? I'm not aware of any that function like this in the base game, but there is an enormous amount of monsters in the game now so there could easily be one.
However if this behaviour for the monster is some kind of homebrew stat-block or ruling that your DM has made then you're not really looking for a rules answer (because Rules As Written is no, it needs to be on 0 hp) but with custom rules at play your DM should decide whether it counts as "dead" for the purposes of switching your mark. Personally I'd say yes, because for all intents and purposes the automaton is "dead", but that's just what I'd do.
I looked up Constructs and there are some (like Animated Armor) which become incapacitated in an antimagic field. But in that case Hunter's Mark would be suppressed as well.
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Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
Being incapacitated and reduced to 0 hit points is two different things i wouldn't expect anything triggering off being reduced to 0 hit points specifically to be so when becoming incapacitated while still having hit points.
Being incapacitated and reduced to 0 hit points is two different things i wouldn't expect anything triggering off being reduced to 0 hit points specifically to be so when becoming incapacitated while still having hit points.
The issue is that if this is not a standard monster, then the normal rules don't necessarily apply, as this would be an unforeseen side effect of a DM's custom behaviour; for the purposes of a homebrew monster that can be taken "offline" without dropping to 0 hit-points, then some allowances should be made based upon whether it's effectively "dead" for the purposes of the campaign.
On the other hand if it's only effectively "asleep" (in the same way as a creature hit by a sleep spell) then the mark should be "stuck" to it until it reboots or whatever, but it's pure DM call as only they know how their monster behaves and why (and under what circumstances it might recover, if at all).
Being incapacitated and reduced to 0 hit points is two different things i wouldn't expect anything triggering off being reduced to 0 hit points specifically to be so when becoming incapacitated while still having hit points.
The issue is that if this is not a standard monster, then the normal rules don't necessarily apply, as this would be an unforeseen side effect of a DM's custom behaviour; for the purposes of a homebrew monster that can be taken "offline" without dropping to 0 hit-points, then some allowances should be made based upon whether it's effectively "dead" for the purposes of the campaign.
On the other hand if it's only effectively "asleep" (in the same way as a creature hit by a sleep spell) then the mark should be "stuck" to it until it reboots or whatever, but it's pure DM call as only they know how their monster behaves and why (and under what circumstances it might recover, if at all).
For such a case to be more specific than the general rule, the monster statblock would have to clearly say so, and construct do not. Any creature becoming unconscious or incapacitated as a result of Antimagic Susceptibility or any other reasons never count as being reduced to 0 hit points whatsoever to meet such requirement, unless the reason they get this condition is directly a result of it.
On the other hand, being reduced to 0 hit point while not unconscious or incapacitated somehow would allow one to move an Hunter's Mark from it, such as a Samurai feature Strength Before Death.
For such a case to be more specific than the general rule, the monster statblock would have to clearly say so, and construct do not. Any creature becoming unconscious or incapacitated as a result of Antimagic Susceptibility or any other reasons never count as being reduced to 0 hit points whatsoever to meet such requirement, unless the reason they get this condition is directly a result of it.
You don't know any of this as we haven't been told what the monster is exactly or how it came to be in this "offline but not at 0 hp" state.
The OP's question isn't even strictly asking what the rules say, but whether people would allow the mark to be switched which is entirely subjective; until we know more it's not really a rules & game mechanics issue to begin with.
The RAW says no because hunter's mark says 0 hp, but it sounds a lot like none of the rest of this is RAW so there's no RAW answer to this situation other than "ask the DM".
As I am the DM, I allowed it to move as the creature was not able to do anything and was effectively dead (Except it had 9 hp) but it was a fun discussion as to the question of could the ranger move it or not.
Personally I think that was the right call; often when DMing what matters most is the intention or spirit of the rules, and I believe the intention with hunter's mark is that you are marking a quarry you intend to hunt down until it is defeated/dying/dead, which for most creatures normally means being at 0 hit-points, which is why it works the way it does. You're not supposed to transfer it to another creature until the currently marked target has been dealt with.
For this automaton (which I'll need to check out) it's effectively defeated once it fails this save for short-circuiting, so it seems reasonable to count that. For all intents and purposes it's basically a corpse already because it's out of the combat and you can then destroy it at your leisure or ignore it entirely if you do not wish to repair it.
Plus being forced to keep attacking an already "dead" robot just so you aren't wasting a spell slot on a half caster would suck for a player; makes little sense mechanically to force that, and less sense narratively.
Another example would be petrification. Effectively the target is incapacitated and unable to do anything unless someone fixes them, BUT they are not on 0hp and they're not really dead.
A petrified person can still be targeted, as could an incapacitated automation.
With this and the RAW wording of Hunter's Mark, I'd argue you cannot switch it... But you're free to keep attacking the incapacitated creature. The fact it's incapacitated without the ability to get back up unless assisted is besides the point. Hunter's Mark is magical and seems to be set to only allow re-allocation when the target is considered to be on 0HP in a damage sense.
Thank you for the suggestions and I think I may have to give you more information on the situation.
To assist in this the stat block says
Whenever the creature starts its turn with 10 or fewer hit points it must make a Constitution Save (DC15). Failure indicates that it has short-circuited and will be incapacitated until repaired.
The creature was at 9hp and failed the save. Thus incapacitated, but still has 9hp.
Hunters Mark says - If the target drops to 0 hp before the spell ends, you can use a bonus action on a subsequent turn of yours to mark a new creature.
As I am the DM, I allowed it to move as the creature was not able to do anything and was effectively dead (Except it had 9 hp) but it was a fun discussion as to the question of could the ranger move it or not.
That was your call and it's fine. Personally my call would be different as creature short-circuited this way are not effectively dead to me, they're simply incapacitated until repaired. As written its not even unconscious and so its still aware of it's surrounding, it only can't take actions or reactions. Ranger can either then drop it to 0 hit point if it want to move its Hunter's Mark, or shift its attention to other enemies while this one is incapacitated.
Technically, you wouldn't be allowed to move Hunter's Mark. Can you tell more about this automaton (a Construct?) and how it was incapacitated?
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
Do you remember what the automaton was called? I'm not aware of any that function like this in the base game, but there is an enormous amount of monsters in the game now so there could easily be one.
However if this behaviour for the monster is some kind of homebrew stat-block or ruling that your DM has made then you're not really looking for a rules answer (because Rules As Written is no, it needs to be on 0 hp) but with custom rules at play your DM should decide whether it counts as "dead" for the purposes of switching your mark. Personally I'd say yes, because for all intents and purposes the automaton is "dead", but that's just what I'd do.
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I looked up Constructs and there are some (like Animated Armor) which become incapacitated in an antimagic field. But in that case Hunter's Mark would be suppressed as well.
Helpful rewriter of Japanese->English translation and delver into software codebases (she/e/they)
Being incapacitated and reduced to 0 hit points is two different things i wouldn't expect anything triggering off being reduced to 0 hit points specifically to be so when becoming incapacitated while still having hit points.
The issue is that if this is not a standard monster, then the normal rules don't necessarily apply, as this would be an unforeseen side effect of a DM's custom behaviour; for the purposes of a homebrew monster that can be taken "offline" without dropping to 0 hit-points, then some allowances should be made based upon whether it's effectively "dead" for the purposes of the campaign.
On the other hand if it's only effectively "asleep" (in the same way as a creature hit by a sleep spell) then the mark should be "stuck" to it until it reboots or whatever, but it's pure DM call as only they know how their monster behaves and why (and under what circumstances it might recover, if at all).
Characters: Bullette, Chortle, Dracarys Noir, Edward Merryspell, Habard Ashery, Legion, Peregrine
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Guides: Creating Sub-Races Using Trait Options
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Please don't reply to my posts unless you've read what they actually say.
For such a case to be more specific than the general rule, the monster statblock would have to clearly say so, and construct do not. Any creature becoming unconscious or incapacitated as a result of Antimagic Susceptibility or any other reasons never count as being reduced to 0 hit points whatsoever to meet such requirement, unless the reason they get this condition is directly a result of it.
On the other hand, being reduced to 0 hit point while not unconscious or incapacitated somehow would allow one to move an Hunter's Mark from it, such as a Samurai feature Strength Before Death.
Characters: Bullette, Chortle, Dracarys Noir, Edward Merryspell, Habard Ashery, Legion, Peregrine
My Homebrew: Feats | Items | Monsters | Spells | Subclasses | Races
Guides: Creating Sub-Races Using Trait Options
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Please don't reply to my posts unless you've read what they actually say.
Personally I think that was the right call; often when DMing what matters most is the intention or spirit of the rules, and I believe the intention with hunter's mark is that you are marking a quarry you intend to hunt down until it is defeated/dying/dead, which for most creatures normally means being at 0 hit-points, which is why it works the way it does. You're not supposed to transfer it to another creature until the currently marked target has been dealt with.
For this automaton (which I'll need to check out) it's effectively defeated once it fails this save for short-circuiting, so it seems reasonable to count that. For all intents and purposes it's basically a corpse already because it's out of the combat and you can then destroy it at your leisure or ignore it entirely if you do not wish to repair it.
Plus being forced to keep attacking an already "dead" robot just so you aren't wasting a spell slot on a half caster would suck for a player; makes little sense mechanically to force that, and less sense narratively.
Characters: Bullette, Chortle, Dracarys Noir, Edward Merryspell, Habard Ashery, Legion, Peregrine
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Guides: Creating Sub-Races Using Trait Options
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Please don't reply to my posts unless you've read what they actually say.
Another example would be petrification. Effectively the target is incapacitated and unable to do anything unless someone fixes them, BUT they are not on 0hp and they're not really dead.
A petrified person can still be targeted, as could an incapacitated automation.
With this and the RAW wording of Hunter's Mark, I'd argue you cannot switch it... But you're free to keep attacking the incapacitated creature. The fact it's incapacitated without the ability to get back up unless assisted is besides the point. Hunter's Mark is magical and seems to be set to only allow re-allocation when the target is considered to be on 0HP in a damage sense.
But a DM is always free to rule otherwise.
That was your call and it's fine. Personally my call would be different as creature short-circuited this way are not effectively dead to me, they're simply incapacitated until repaired. As written its not even unconscious and so its still aware of it's surrounding, it only can't take actions or reactions. Ranger can either then drop it to 0 hit point if it want to move its Hunter's Mark, or shift its attention to other enemies while this one is incapacitated.