From Latin 'in-' (over) + 'pendere' (to hang) — the image of something dangling overhead became the metaphor for imminent events.
To be about to happen, especially something threatening or momentous; to hang over or loom threateningly.
From Proto-Indo-European *(s)pen- ("to draw, stretch, spin"), through Latin impendere ("to hang over, to threaten, to be imminent"), composed of in- ("upon, over") + pendere ("to hang"). The PIE root *(s)pen- carried the sense of something suspended by tension, which developed in Latin into pendere ("to hang") and pendere ("to weigh, pay"). The metaphor of something hanging overhead as a threat is ancient
The Sword of Damocles — a single sword hung by a horsehair over a courtier's head during a feast — is the perfect image of what 'impend' literally means. The sword impended: it hung over Damocles, threatening to fall at any moment. Every time English speakers say 'impending doom,' they are unconsciously invoking that ancient image of something dangerous suspended overhead.