Lutum is Amber's homeworld. It was originally a volcanic wasteland with primitive life, until it's moon, Pluvium, passed the Roche limit around 40 thousand years ago.
Since then, whenever the moon completes it's orbit parts of its icy crust bombard the planet below and initiate the downpour. A planet-wide rainstorm lasting between 3 to 10 days. This causes sea level to rise about 2cm a year, meaning that eventually the planet will be fully underwater. That will take over 100 thousand years though.

Life on the planet exploded once the rain started falling, eventually creating the modern Sawtooth Locust. Every year, they return in the quadrillions, starting the Swarm Season. It is dangerous to venture outside during this season, however the recently native kobold's have adapted to these conditions with bug proof fabric and nets.
But once the swarms come, the Worms soon follow. Giant worms known as God Worms live in the deep crust of Lutum and surface to feed during the swarm season. The downpour always marks the end of the swarm season.