Thunderstorm

You raise your hands, and storm clouds gather. Rain starts to fall, making everybody but you wet near instantly and putting out torches. Lightning flashes, thunder roars. You have summoned a thunderstorm.

The storm's center is on you, and it extends through the sky for 100 feet in all directions. If you are not outdoors, then the storm is still created with you at it's center, but it won't do anything. You can also use this spell to take control of a naturally occurring thunderstorm.

This spell is unusual, because the caster can pour more of their magical energy into it than other similarly strong spells. In game terms, you use other spell slots to make it stronger. Depending on what level spell slot you use, you can power your spell up in different ways. 

Level 1 Spell Slot: by using a level 1 spell slot on Thunderstorm, you call down lightning on a target under the rain clouds. The target must make a dexterity save or take 8d6 lightning damage, half if they succeed.

Level 2 Spell Slot: a level 2 spell slot temporarily adds hail to the precipitation. Anybody out in the open (aside from you) takes 1d6 bludgeoning damage per minute. The hail lasts for 2 minutes.

Level 3 Spell Slot: a level 3 spell slot adds wind to the storm. The wind makes the air count as difficult terrain for any flying creature's. If you take flight, the wind will not affect you. You pick the direction it blows. It lasts for as long as the storm is up. 

Level 4 Spell Slot: you make the rain fall harder. Much harder. So much, that it starts pooling on the ground. If there are any campfires (or similarly sized flames), they are put out by this torrent. The ground is now difficult terrain because it is slippery. You are immune to the slippery ground. If it is hailing while you do this, add a d6 of bludgeoning damage. 

Level 5 Spell Slot: the rain becomes cold. Frigid, even. Anybody out in the open will take 1d6 cold damage. If you have increased the rainfall (level 4), they take 2d6 cold damage instead. You are immune to the freezing rain. 

Level 6 Spell Slot: the storm becomes more ferocious. You double all damage that you do with this spell. Not by doubling the amount of dice rolled, but by rolling any dice and then doubling the number. 

Level 7 Spell Slot: if you use a level 7 spell slot, the clouds darken further. Almost no light makes it through the clouds. The area has the effects of dim lighting during the day, and is completely dark. This does affect you. 

Level 8 Spell Slot: by using a level 8 spell slot, you can either double the diameter of the storm, or you can keep it up for another hour. 

In combat, flip a coin after each of your turns. If it's tails, then nothing happens. If it's heads, lightning strikes. This is out of your control, and doesn't use a level 1 spell slot. If lightning randomly strikes, roll a d8. If it's a 1-2, the lightning strikes a random ally. If it's a 3-4, then it strikes the ground and doesn't hit anybody. If it's a 5-6, then it strikes a random creature. Enemy, ally, or neutral, it doesn't matter. If it's a 7-8 then it strikes a random enemy.