Uranium235
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It's funny that all Hellkites are translated as snakes.
Although a snake is a serpent, and hellkite is still something slightly different; hell means hell, and kite means a kite (noun) or kite (verb) - to soar in the air, and the second one fits the context much better.
P.S. - and specifically, hellkite means a villain or a fiend. I like the translation "fiend," as it fully reflects the essence of all these dragons.