Dragon, Shadow Ancient (CR 21)

Huge Dragon
Alignment: Always chaotic evil
Initiative: +0; Senses: blindsense 60 ft., darkvision120 ft., low-light vision, and keen senses
Languages: Draconic

AC: 42 (-2 size, +34 natural), touch 8, flat-footed 42
Hit Dice: 31d12+186 (387 hp); DR: 15/magic
Fort +22, Ref +17, Will +26
Speed: 80 ft., fly 150 ft. (poor)
Space: 20 ft./10 ft.
Base Attack +31; Grapple +49
Attack: 1 bite +39, 2 claws +34, 2 wings +34, 1 tail slap +34, 1 crush +34
Damage: 1 bite 2d8+10, 2 claws 2d6+5, 2 wings 1d8, 1 tail slap 2d6, 1 crush 2d8
Special Attacks/Actions: Breath weapon 6(DC 34)
Abilities: Str 31, Dex 10, Con 23, Int 28, Wis 28, Cha 29
Special Qualities: Energy drain immunity, Shadow blend, Mirror Image, Dimension door, Nondetection, Shadow walk, Frightful Presence DC34, SR 30
Feats: #Feats: 11
Skills: Skill points: 285
Advancement: 32-33 HD (Gargantuan)
Climate/Terrain: Underground
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 shadow dragons breath weapon is a cone of billowing, smoky shadows with an energy drain effect. Creatures within the cone gain 6 negative levels: the saving throw to remove the negative level is 36. A successful Reflex save (DC 36) reduces the number of negative levels by half (round down).

Shadow Blend (Su): During any conditions other than full daylight, a shadow dragon can disappear into the shadows, giving it nine-tenths concealment. Artificial illumination, even a light or continual flame spell, does not negate this ability. A daylight spell, however, will.