Hail

Biome

-Mostly happens in snowy biomes, but it will sometimes happen in forest and plains.

Features

-On hard mode, it has a chance to change any block it hits to ice. 

-It can do damage to entities and act like a combination of an anvil and powdered snow. In other words it will inflict both cold and height damage.

-It'll give the entity nausea or blindness for 5 seconds.

If... 

-If hail hits a campfire the fire will go out.

-If it hits water, the surrounding water (8 block radius) will do cold damage for 45 seconds.

-If it hits ice or glass it will shatter it.

-If it hits lava it will reduce the surrounding lava's (8 blocks again) damage by 50%

-If it somehow hits a blaze or any nether mob it will be stunned for 10 seconds and will take more damage than a normal entity.

-If it is hit by a projectile it will split. 

-If it happens at night the damage will be higher.

That's all I have but any extra suggestions will be appreciated in the comments!