Dragon, Rust Very Old (CR 20)

Gargantuan Dragon (Extraplanar)
Alignment: Always lawful evil or neutral
Initiative: +0; Senses: blindsense 300 ft., darkvision 1000 ft., and low-light vision
Languages: Draconic

AC: 39 (-4 size, +33 natural), touch 6, flat-footed 39
Hit Dice: 33d12+231 (445 hp); DR: 20/magic and 5/chaotic
Fort +25, Ref +18, Will +20
Speed: 60 ft., fly 200 ft. (clumsy), burrow 45 ft.
Space: 20 ft./15 ft. (20 ft. with bite)
Base Attack +33; Grapple +57
Attack: 1 bite +41, 2 claws +36, 2 wings +36, 1 tail slap +36, 1 crush +36, 1 tail sweep +36
Damage: 1 bite 4d6+12, 2 claws 2d8+6, 2 wings 2d6, 1 tail slap 2d8, 1 crush 4d6, 1 tail sweep 2d6
Special Attacks/Actions: Breath weapon 20d4 (33)
Abilities: Str 35, Dex 10, Con 25, Int 14, Wis 15, Cha 14
Special Qualities: Metal resistance, rusting bite, Wall of iron, Acid Fog, Repel metal or stone, SR 30, Fear aura (DC 30)
Feats: #Feats: 12
Skills: Skill points: 72
Advancement: 34-35 HD (Gargantuan)
Climate/Terrain: Any land and underground (Acheron)
Organization: Solitary (1 dragon, any age), clutch (1d4+1 wyrmlings, very young, young, juvenile, or young adults), family (pair of mature adults and 1d4+1 offspring)
Treasure/Possessions: Triple Standard

Source: Draconomicon

Breath Weapon (Su): A rust dragon has two breath weapons: a line of acid or a cone of reddish-brown liquid that instantly corrodes and destroys any metal it touches. Attended and magical metals receive Reflex saves to avoid this effect, but any metal is susceptible: iron, steel, silver, gold, even mithral and adamantine.

Metal Resistance (Ex): A rust dragon is resistant to attacks from metal weapons. Against weapons whose damage-dealing part is metal (a blade, metal point, arrow-head, or even mace head), a rust dragon has damage reduction equal to what a rust dragon two age categories older than itself has. Wyrm and great wyrm rust dragons have damage reduction 20/magic and 10/chaotic against metal weapons, and lesser weapons corrode when used against them (see Rusting Scales, below).

Rusting Bite (Ex): A rust dragon that makes a successful bite attack causes metal armor worn by the target creature to fall to pieces and becoming useless immediately. A dragon can also use its bite attack to target a weapon or other metal object, of course. The size of the object is immaterial - a full suit of armor rusts away as quickly as a sword.

Magic metal items are allowed Reflex saves against a DC equal to the dragon's breath weapon save DC.

Rusting Scales (Ex): A metal weapon with less than a +5 enhancement bonus that hits a wyrm; or great wyrm rust dragon corrodes and is destroyed immediately, with no saving throw. A +5 weapon deals damage normally, but then must succeed on a Reflex save (DC equal to the dragon's breath weapon save DC) or rust away.

Spell-Like Abilities: 3/day - wall of iron; 1/day - acid fog, repel metal or stone.

Frightful Presence (Ex): A very old adult dragon can unsettle foes with its mere presence. The ability takes effect automatically whenever the dragon attacks, charges, or flies overhead. Creatures within a radius of 270 feet are subject to the effect if they have fewer HD than the dragon (30).

A potentially affected creature that succeeds at a Will save (DC 27) remains immune to that dragon's frightful presence for one day. On a failure, creatures with 4 or fewer HD become panicked for 4d6 rounds and those with 5 or more HD become shaken for 4d6 rounds.

Dragons ignore the frightful presence of other dragons.

Immunities (Ex): All dragons are immune to sleep and paralysis effects. Rust dragons are also immune to acid.

Spell Resistance (Ex): As dragons age, they become more resistant to spells and spell-like abilities.

Blindsight (Ex): A dragon can ascertain creatures by non-visual means (mostly hearing and scent, but also by noticing vibration and other environmental clues) with a range of 270 feet.

Keen Senses (Ex): A dragon sees four times as well a human in low-light conditions and twice as well in normal light. It also has darkvision with a range of 900 feet and scent ability to 270 feet.

Extraplanar Subtype

A subtype applied to any creature when it is on a plane other than its native plane. A creature that travels the planes can gain or lose this subtype as it goes from plane to plane. This book assumes that encounters with creatures take place on the Material Plane, and every creature whose native plane is not the Material Plane has the extraplanar subtype (but would not have when on its home plane). An extraplanar creatures usually has a home plane mentioned in its description. These home planes are taken from the Great Wheel cosmology of the D&D game (see Chapter 5 of the Dungeon Master's Guide). If your campaign uses a different cosmology, you will need to assign different home planes to extraplanar creatures.

Creatures not labeled as extraplanar are natives of the Material Plane, and they gain the extraplanar subtype if they leave the Material Plane. No creature has the extraplanar subtype when it is on a transitive plane; the transitive planes in the D&D cosmology are the Astral Plane, the Ethereal Plane, and the Plane of Shadow.