Dragon Alternate Form feature vs Draconic Polymorph

Question

I was curious about what the differences are for a True Dragon between using its natural Alternate Form feature vs the Draconic Polymorph spell.

The Alternate Form feature (link to Gold Dragon on SRD) says this:

Alternate Form (Su): A gold dragon can assume any animal or humanoid form of Medium size or smaller as a standard action three times per day. This ability functions as a polymorph spell cast on itself at its caster level, except that the dragon does not regain hit points for changing form and can only assume the form of an animal or humanoid. The dragon can remain in its animal or humanoid form until it chooses to assume a new one or return to its natural form.

The Draconic Polymorph spell description (from the Draconomicon) says this:

As polymorph, except as follows: The assumed form can have no more Hit Dice than you have, to a maximum of 20 Hit Dice. Your Strength and Constitution in the new form are above the average for the race or species into which you transform; your Strength is the average for the creature +8, and your Constitution is the average for the creature +2.

So, I have a question about this:

Damage: Does a dragon using the Alternate Form feature only revert to its true form if it takes enough damage to kill said true form, or might it revert to true form if it takes enough damage to kill its assumed form?

Answer 1

If you use Alternate Form or the (Draconic) Polymorph spell you gain the physical ability scores (Str, Dex, Con) of the new form (which get modified in case of Draconic Polymorph). Normally, when an ability score changes, all attributes associated with that score change accordingly. Thus, if your CON modifier changes, your number of hit points change.

But this general rule does not apply to the supernatural ability Alternate Form.

"Apply any changed physical ability score modifiers in all appropriate areas with one exception: the creature retains the hit points of its original form despite any change to its Constitution."

So concerning hit points, there is no difference between the assumed form and the true form. An ancient gold dragon who uses Alternate Form to change into a halfling still has 542 hp.

If you use the spell Draconic Polymorph (Draconomicon, 79) (which is just a stronger version of the Polymorph spell (and has actually nothing to do with dragons)) it is controversial whether your hit points remain unchanged or whether they should be recalculated using the constitution modifier of your new form (see Q&A here and here and also Yopi Lapis comments below). According to the rules of the Polymorph Subschool any target of a spell from that school also retains its own hit points. But then again, the specific rules of the spells Alter Self, Polymorph (and Draconic Polymorph) and Shapechange trump the general rules of that school.

No matter whether you changed form via Alternate Form or the Polymorph spell: only the stats of your assumed form are relevant for anything that affects you and for any damage you take. If your assumed form drops to -10 hp you die and revert to your original form.

Alternate Form: "A creature using alternate form reverts to its natural form when killed"

Polymorph: "If slain, the subject reverts to its original form, though it remains dead."