There is no "hellion" in Greek myth.
If by chance you mean Hêlios, the Greek Titan god of the Sun; no he was not "bad"; he was the guardian of oaths and the god of gift of sight (prophesy). Every day he guided the chariot of the sun across the sky.
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A primordial Chasm, from which several "bad" things descended.
There is no Eric in Greek mythology.
Boltar is not from Greek mythology.
Greek Mythology is older.
I apologize, but Istra is NOT in Greek Mythology. Sorry!!