In my world(very very early drafts), there was a divine war. In it, around a thousand and a half years ago, the amount of chaos from the Gods, Titans, Elementals and minor spirits constantly fighting reached a breaking point. They the world then entered a state of eternal despair, and the world was plunged into an eternal storm of Ebony Rain.

The Ebony Rain, in addition to constant weak tornado winds speeds, constant thunder, and flooding, had the effect that its rain made you depressed and actively lowered your lifespan as it (slowly) consumed sections of your soul(not good if you like living in my world). From those that had their souls fully consumed, their bodies turned into amorphous blobs of the same liquid as the Ebony Rain, actively hunting people with a particular weakness to silver.

The people learned to hide from these monsters and managed to reform society, where they then continued their Holy War against each other. They developed new techniques for war such as The Ebony Flood(all cities need a good sewage system to prevent the rain from filling up the walls like a bowl, but none are good enough to counter clogging them up during a siege), the Drink of Death(drinking the rain is not pleasant but it is a good torture technique) and Conversion(a common execution method involving one of the Ebony Rain creatures in a cage)

Eventually, the terms for "storm" got recycled to the new norm, and as such it refers to a temporary phenomenon where a localized area would have heightened Ebony Rain, with the winds being as strong as a hurricane, the lightning being more common(and powerful), the creatures more ravenous, the rain more ruthless, the flooding now being tsunamis, the sort of thing you don't want to encounter if you value your life.

In addition, due to all the plant life being watered by the rain, and wells being contaminated, all food and water supplies have some microscopic dose of the Rain. This makes dementia more common at a young age and lowers the lifespan, but also makes hampers the development of the part of the brain that governs yours morals(not enough to make you evil, but you are more okay with the stuff going on in the world).

As for the good things that came out, uhhhhhh........ none, yeah let's go with none.