Do Red Dragon Disciples become vulnerable to cold?

Question

Basically, I'm asking if Dragon Disciples receive the elemental vulnerability of their dragon ancestor.

And what about Half-Dragons?

The "Red" in "Red Dragon Disciples" refers to their "red dragon heritage".

Answer 1

A Dragon Disciple does not gain any vulnerabilities directly from his class, see the SRD on Dragon Disciple. However, at level 10 a Dragon Disciple gains the Half-Dragon Template, which states (emphasis mine):

Special Qualities

A half-dragon has all the special qualities of the base creature, plus darkvision out to 60 feet and low-light vision. A half-dragon has immunity to sleep and paralysis effects, and an additional immunity based on its dragon variety.

The specific dragon that is related to the dragon disciple is extremely important. For instance, a red dragon disciple would gain fire immunity but a fang dragon disciple would gain no immunity (Draconomicon, page 167).

However, there is no mention of gaining a vulnerability anywhere in either the dragon disciple class or half-dragon template, so I would answer both of your questions with 'No'.

Answer 2

Yes, they become vulnerable.

Cold immunity (Dungeon Master's Guide 2003, 291) is a specific ability, as is fire immunity (294). In each description, the immune creature has vulnerability to the opposing element.

A creature with cold immunity, such as a frost giant, never takes cold damage. It has vulnerability to fire, which means it takes half again as much (+50%) damage as normal from fire, regardless of whether a saving throw is allowed, or if the save is a success or failure.

A creature with fire immunity, such as a fire giant, never takes fire damage. It has vulnerability to cold, which means it takes half again as much (+50%) damage as normal from cold, regardless of whether a saving throw is allowed, or if the save is a success or failure.

While it appears, due to the examples of frost and fire giant given, that the writers might, possibly, have been conflating cold, and fire, immunity with the corresponding cold and fire subtypes (Monster Manual, 307, 309), they still made rules independent of those subtypes. (I suspect that those rules show up in the Dungeon Master's Guide rather than the Monster Manual due to the Dragon Disciple specifically, but have no real evidence to back that up.)

So, those two elemental immunities also come with vulnerability to their opposite element, just as those elemental subtypes do. Even though Red, White, Brass, Gold and Silver Half Dragons (Monster Manual, 146-7) and by extension similar Dragon Disciples (Dungeon Master's Guide, 183-5) don't actually gain the cold or fire subtype, their cold or fire immunity still makes them more vulnerable to the opposing element.

Black, Copper, Bronze and Blue half-dragons and dragon disciples would not appear to incur similar vulnerabilities, as their immunities to acid or electricity are not singled out in this way.

Answer 3

True dragons do not inherently have elemental vulnerabilities. Red dragons, brass dragons, and gold dragons are vulnerable to cold because they are (Fire) creatures. White dragons and silver dragons are vulnerable to fire because they are (Cold) creatures.

A creature with the cold subtype has immunity to cold. It has vulnerability to fire, which means it takes half again as much (+50%) damage as normal from fire, regardless of whether a saving throw is allowed, or if the save is a success or failure.

The half-dragon and dragon disciple do not inherit the elemental subtype of their ancestor.

Answer 4

Fire Subtype: A creature with the fire subtype has immunity to fire. It has vulnerability to cold, which means it takes half again as much (+50%) damage as normal from cold, regardless of whether a saving throw is allowed, or if the save is a success or failure.

• Monster Manual III, page 213
• Monster Manual IV, page 210
• Monster Manual V, page 211

Since those are more recent than the DMG and the MM, I'd assume these would overrule it.