Your first world:

When you load into a world the first time with the mod, after about 5min the first thing you will see is a white screen with a tinnitus effect and a camera shake. This is an indication, that a meteor has landed near you which introduces the main threat to you.

Meteors:

The meteors spawn the death angels and bring them into your world and land every 10 minutes (12000 ticks). They land within a 70 block radius of your character when the 10 minute cooldown is finished. There will be an audible, static-like sound as the meteor enters the atmosphere which is seen as a huge fireball hurtling to the ground. Once it lands the screen turns white and shakes with a tinnitus sound playing; however some of these effects can be bypassed by being underground or in shelter. For example if you go underground deep enough you will only get a screen shake effect removing the other 2 entirely.

When a meteor lands there are 2 mobs that can spawn from it. The Standard Death Angel (80% chance) and the Queen Death Angel (20% Chance). Although it isn't only the mob that spawns that differs with these meteors as they also differ visually.

The queen meteors differ from the standard meteors not only visually but also spreads the mycelium and mushrooms throughout the world.

Gamerules for meteors: 

The spawn rate can be changed by changing the game rule meteorSpawnRate which uses ticks as the value so, for example, doing /gamerule meteorSpawnRate 24000 will make it so that after the next meteor lands they will land every 20 minutes (or 24000 ticks).

The meteors can also be disabled completely through the game rule allowmeteorspawn which can be turned on and off through setting it to true or false.

The flash produced from the meteors can also be disabled through the game rule doMeteorFlash which is also disabled through true or false.