They originally appeared in the Monstrous Manual for AD&D 2nd edition, I believe.
@Eyes of Nine, yep it looks like Real is correct. From the 2e Monstrous Compendium
Dragon, Mercury
2140
| Climate/Terrain: | Temperate and subtropical/Mountains |
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| Frequency: | Very rare |
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| Organization: | Solitary or clan |
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| Activity Cycle: | Any |
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| Diet: | Omnivore |
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| Intelligence: | Highly (13-14) |
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| Treasure: | See below |
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| Alignment: | Chaotic good |
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| No. Appearing: | 1 (2-5) |
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| Armor Class: | -1 (base) |
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| Movement: | 15, Fl 36 (C), Jp 3 |
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| Hit Dice: | 11 (base) |
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| THAC0: | 9 (base) |
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| No. of Attacks: | 3 + special |
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| Damage/Attack: | 2-8/2-8/2-20 |
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| Special Attacks: | See below |
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| Special Defenses: | Variable |
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| Magic Resistance: | See below |
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| Size: | H (25' base) |
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| Morale: | Fanatic (17-18) |
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| XP Value: | Variable |
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Mercury
dragons are fast, highly-maneuverable creatures with relatively small bodies and long tails. Although good in alignment, they are very whimsical, making and changing decisions frequently.
At birth, a mercury dragon's scales are dull silver. As it ages, the scales become brighter and brighter, until at adult age they gain a brilliant mirror finish. Sunlight or other sources of light reflecting off the scales and wings of a mercury dragon can be blinding.
Mercuries speak the language of good dragons, but at high speed, so there's only a 75% chance of understanding a mercury dragon.
Combat: Mercury dragons are as unpredictable when it comes to combat, as they are in any other situation. They may parley, they might attack instantly, or, perhaps, they may avoid combat entirely. They never attack good-aligned creatures unless sorely provoked.
Mercury dragons always use spells in combat, if possible. They are very creative, and can always figure out some innovative way of using virtually any spell to advantage in combat.
In addition to the breath weapon and the attack modes shared by all dragons, mercury dragons can curve the mirror-bright membranes of their wings to reflect and concentrate available light (as dim as full moonlight) into a beam of dazzling brightness. They can aim the beam at one enemy per round - at the expense of not being able to use their wing buffet, and the enemy must roll a successful saving throw vs. spell or be blinded for 1d4+1 rounds. If not using this technique as a weapon, they can use the beam much like a search-light.
A mercury dragon's breath weapon is a beam of brilliant, yellow light. The beam is 5 feet wide and extends 60 feet from the creature's mouth. Any creature caught in the beam receives damage from heat (saving throw for half damage). The heat of the beam is intense enough to ignite flammable objects that fail saving throws vs. magical fire.
A mercury dragon casts spells and uses magical abilities at the 10th level, plus its combat modifier. At birth, mercury dragons are immune to fire and all magical forms of blindness. They also receive a +3 bonus to saving throws against light-based attacks. As they age, they gain the following additional powers:
Young: gaze reflection at will;
Juvenile: mirror image three times per day;
Adult: polymorph self twice per day;
Old: telekinesis twice per day;
Wyrm: project image once per day
Habitat/Society: Mercury dragons are loners by nature. Their mating behavior is free-wheeling, fun loving, and generally irresponsible. If a female becomes impregnated, however, the male's protective instincts take over. Mercuries are very protective of their offspring, and will give their lives to save them. Offspring usually stay with their parents until they reach the
juvenile age category.
Because of their unpredictable, sometimes almost irrational nature, mercuries very rarely have close relationships with other creatures in the area. For this reason, mercuries have to depend on magical and mechanical traps and guards to protect their lairs when they are away.
Ecology: Mercury dragons eat anything, but they prefer to feed on metal ores. Although they have no venom attacks, the flesh of mercury dragons is highly poisonous.
| Age | Body Lgt. (') | Tail Lgt. (') | AC | Breath Weapon | Spells W | MR | Treas. Type | XP Value |
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| 1 Hatchling | 3-6 | 3-6 | 2 | 2d8+1 | Nil | Nil | Nil | 1,400 |
| 2 Very young | 6-11 | 6-11 | 1 | 4d8+2 | Nil | Nil | Nil | 3,000 |
| 3 Young | 11-17 | 11-20 | 0 | 6d8+3 | Nil | Nil | Nil | 5,000 |
| 4 Juvenile | 17-21 | 20-25 | -1 | 8d8+4 | 1 | 10% | ½H | 8,000 |
| 5 Young adult | 21-24 | 26-30 | -2 | 10d8+5 | 1 1 | 15% | H | 10,000 |
| 6 Adult | 24-27 | 30-33 | -3 | 12d8+6 | 2 1 1 | 20% | H | 11,000 |
| 7 Mature adult | 27-30 | 33-36 | -4 | 14d8+7 | 2 2 2 | 25% | H | 12,000 |
| 8 Old | 30-33 | 36-39 | -5 | 16d8+8 | 3 2 2 1 | 30% | H,I | 14,000 |
| 9 Very old | 33-36 | 39-42 | -6 | 18d8+9 | 3 3 2 2 | 35% | H,I | 15,000 |
| 10 Venerable | 36-39 | 42-45 | -7 | 20d8+10 | 3 3 3 2 1 | 40% | H×2,I | 16,000 |
| 11 Wyrm | 39-41 | 45-48 | -8 | 22d8+11 | 3 3 3 2 2 1 | 50% | H×2,I,X | 17,000 |
| 12 Great Wyrm | 41-44 | 48-51 | -9 | 24d8+12 | 3 3 3 3 2 2 1 | 70% | H×3,I,T,X | 18,000 |
