Control Weather allows a single caster to control a large area and impose life threatening conditions on a significant number of people at once
Control Weather states:
You take control of the weather within 5 miles of you for the duration. You must be outdoors to cast this spell. Moving to a place where you don’t have a clear path to the sky ends the spell early.
You can change precipitation, temperature, and wind. It takes 1d4 × 10 minutes for the new conditions to take effect. Once they do so, you can change the conditions again.
When you change the weather conditions, find a current condition on the following tables and change its stage by one, up or down. When changing the wind, you can change its direction.
- Unbearable Heat
- Arctic Cold
The options for temperature are the ones we need to look at. There are 6 steps of temperature, two of which are life threatening to people within a 10 mile diameter circle centred on the caster. As a result it takes a maximum of two changes for a caster to move the weather to a life-threatening temperature. At most this requires 80 minutes of time for these changes to take effect.
The DMG, in the Wilderness Survival section lists the effects of extreme temperatures:
Whenever the temperature is at or below 0 degrees Fahrenheit, a creature exposed to the cold must succeed on a DC 10 Constitution saving throw at the end of each hour or gain one level of exhaustion. [...]
When the temperature is at or above 100 degrees Fahrenheit, a creature exposed to the heat and without access to drinkable water must succeed on a Constitution saving throw at the end of each hour or gain one level of exhaustion. The DC is 5 for the first hour and increases by 1 for each additional hour. Creatures wearing medium or heavy armor, or who are clad in heavy clothing, have disadvantage on the saving throw. [...]
Either of these two weather conditions being imposed has the potential to kill or incapacitate a significant number of people in a comparatively short space of time.
In addition to those two deadly effects, the caster can also impose, in a maximum of 80 minutes, strong winds (in parallel to the temperature changes), and in a maximum of 160 minutes heavy precipitation.
The same DMG section describes these effects as:
A strong wind imposes disadvantage on ranged weapon attack rolls and Wisdom (Perception) checks that rely on hearing. A strong wind also extinguishes open flames, disperses fog, and makes flying by nonmagical means nearly impossible. A flying creature in a strong wind must land at the end of its turn or fall.
A strong wind in a desert can create a sandstorm that imposes disadvantage on Wisdom (Perception) checks that rely on sight.
Everything within an area of heavy rain or heavy snowfall is lightly obscured, and creatures in the area have disadvantage on Wisdom (Perception) checks that rely on sight. Heavy rain also extinguishes open flames and imposes disadvantage on Wisdom (Perception) checks that rely on hearing.
Using this spell a single caster can shut down non-magical flight, make ranged weapon attacks be taken at disadvantage, give disadvantage on all perception checks (both sight and hearing), and impose significant levels of exhaustion on a huge number of people (eg Waterdeep and its surrounding environs).
Looking at the Tiers of Play described in the PHB the third tier is described as:
In the third tier (levels 11–16), characters have reached a level of power that sets them high above the ordinary populace and makes them special even among adventurers. At 11th level, many spellcasters gain access to 6th-level spells, some of which create effects previously impossible for player characters to achieve.
These mighty adventurers often confront threats to whole regions and continents.
This tier of play fits a character who is facing a threat facing a "whole region" or "continent". This puts the appropriate level of a spell like this somewhere between 6th level and 8th level given its potential for regional dominance.
How do I apply this to my game?
You have expressed an interest in having the NPC be able to control the weather for their village. Given the analysis above, there are some easy tweaks we can make to produce a significantly lower level "Control Local Weather" spell:
- Bring down the radius from 5 miles to 1 mile or even 0.5 miles
- Reduce the spells duration (a 1 hour duration for example would make the extreme weather options infeasible)
- Remove the two extreme temperature options from the possible weather states
- Limit the spell to only one of the two extreme precipitation and wind effects
It will depend on what you want your NPC to do, but applying all of the above could easily reduce the level of the spell down to either tier 2 or even tier 1.
In the second tier (levels 5–10), characters come into their own. Many spellcasters gain access to 3rd-level spells at the start of this tier, crossing a new threshold of magical power with spells such as fireball and lightning bolt. [...] These characters have become important, facing dangers that threaten cities and kingdoms.
In the first tier (levels 1–4), characters are effectively apprentice adventurers. [...] The threats they face are relatively minor, usually posing a danger to local farmsteads or villages.