Phthonos was the personification of jealousy and envy, especially in matters of love. He was said to be the counterpart of Nemesis. The god had a great many wives but killed each one eventually, on suspicions of adultery. However, his power influenced not only he and mortals, but the gods as well (most famously Hera), he is the son of Nyx and Dionysus.
It was he that directed Hera‘s attention to Zeus‘s many affairs, including one of his most famous, that with the Theban princess Semele. He also put the thought into Hera‘s mind to trick Semele into convincing Zeus into showing Semele his full glory, which killed her.