D&D Monsters: Metallic Dragons

In European myth, dragons are almost universally regarded as evil - rapacious monsters that lay waste to the countryside, have to be slain by heroic knights and, in many cases, essentially breathe hellfire. They are, sometimes literally, diabolic creatures. The same is not necessarily true in other cultures, such as that of China, where dragons may not necessarily be easy to parlay with or recruit as allies, but aren't fundamentally hostile, either.

In D&D, the good counterparts to the evil chromatic dragons are, of course, the metallic ones. Indeed, they are among a relatively small number of 'good monsters' to make it consistently through into the core books of later editions. Up until 5E, they are portrayed as rarer, but individually more powerful than, the chromatic dragons. They are perhaps even rarer in games than they are usually described as being in the universe (in two campaigns of Critical Role, the PCs have so far encountered at least seven chromatic dragons, and only one metallic). Doubtless, this is because they are less useful in a typical game if you're not going to fight them - and they're too powerful to be regular allies.

D&D Monsters: Blue Dragons

A considerable number of mythological deities are said to throw thunderbolts - Zeus and Thor are merely the most familiar of these to Europeans, with examples known from many other cultures. Actual mythic creatures that throw lightning, however, are much less common, although Chinese dragons are at least associated with thunder and storms. In D&D, however, it seems an obvious attack mode once we've dealt with fire and ice, and, naturally enough, it's associated with the dragon that's the colour of the sky.

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1E

The original picture of the blue dragon shows an animal with a moderately long and heavy snout, large eyes, and elongated canine teeth. More distinctive features include the fact that the dorsal frill of the green and black dragons is here replaced with a series of large triangular plates, which may be projections from the vertebrae, but could be separate structures similar to those on a Stegosaurus. The latter possibility is supported by the existence of three similar plates on the forehead. In front of these, there is a row of four  bony spikes, one of which is enlarged to form a narrow nasal horn. As usual, this doesn't look like it would be of much use in combat, at least compared to the teeth and claws, so it might be a display structure.