The end of the world happens slowly and gradually instead of one apocalyptic day

This is different from post-apocalyptic because this takes place during that slow destructive event before the end, not after it

Honorable mention: Climate change/Holocene Extinction/Now (IRL). I'm retrospectively adding this because so many of you commented that. Come on, I know, you know, we all already know, so give us something from fiction instead

1. The Trench Crusade (Trench Crusade). In 1099, a bunch of templars opened a cursed artifact that opened the gates of hell and allowed its forces to invade earth. The war between the faithfuls and heretics rages on for over 800 years after, with the faithfuls losing more and more land over time and by 1914, most of the Middle East, North Africa, and parts of the Balkan region, Central Europe, southern Spain, northern Finland, and other places falling to the demons, with more happening everyday. (Correct me if I'm wrong that's what I remember from the last time I saw the map)

2. The Blight (Interstellar) is a fungal infection that affects crops worldwide. Types of crops fails and are no longer able to be grown, and causing food supplies to dwindle globally.

3. The Permian-Triassic Extinction (IRL) was caused when major volcanic activities in Siberia spews out lava, toxic chemicals and greenhouse gasses for millions of years, heating up the earth causing 90-95% of all organisms to die out. Every single thing alive right now is descended from those few that managed to survive