More than a century ago, the old world ended in fire, death, and destruction.
The old Gods were killed, and the old magics became unpredictable and dangerous. Fire rained from the sky, savage monsters ravaged, disease and starvation were everywhere. Rivers and seas left their banks and the weather itself changed everywhere, summer became endless winter, crops and beasts failed and died. Kingdoms fell, uncounted thousands and thousands suffered and died, and all social order disintegrated.
Some exceptional men and women refused to accept death, and they led their families, clans, or small bands of followers. They migrated, they defeated monsters, they survived, and they willed their followers to survive.
Eventually these exceptional leaders died, usually after unnaturally long and amazing lives. Their actions, and the stories told about them and passed down from one generation to the next, continued to inspire their followers. Soon, a few of the followers of these departed heroes started to receive dreams and visions from the dead leaders. Dreams then became miracles, then divine spells and powers, usually granted to the descendants of the revered leader.
They became the New Gods, or the Small Gods. These infant Gods only have a few priests able to channel divine power, less than ten, usually three, two, or one. Most Small Gods have at most a few hundred followers, with a few of the luckiest or those living in the most fertile areas numbering above a thousand.
Each valley, each hill, each stubborn little village, full of stubborn survivors, has its own patron deity. Each clIn life, most of the Small Gods were clannish, tough survivors. Their followers follow their lead, and human institutions remain small and fiercely tribal.
Basically, something caused the world to stop spinning in it's axis. The same thing, or maybe something else, killed or drove off the first Gods.
Weather and the seasons went crazy for a dozen years, society collapsed, and 90-95% of the population died.
After the world settled down again the polar ice caps were in the tropics, the tropics were freezing over, enormous new mountain ranges had exploded out of the earth in as little as a single year, monstrous tides and tsunamis had washed away the ruins of whole kingdoms, and the only remaining survivors were a few hundred clans and small tribes, led personally by their own God and one or two priests.