I am creating a world where exist 2 main types of magic users: the ones with the ability to control the light of the sun and the ones with the ability to control the darkness of the shadows/moon. However, I'm having trouble coming up with a 3rd type of user. This 3rd kind is meant to be stronger than those above, and should have a similar theme. Is there a type of magic, preferably related to the stars, which is classically known to either encompass the previous 2 or be simultaneously opposite to them? The selection here is made through the one which seems to make the most sense within the context of these 3 main sources: light, darkness and [redacted] , with [redacted] being normally considered as above/ superior to the previous 2.
Light is defined by where it is not. If everything is the same brightness, nothing can be seen. Shadow is defined by the light. With no light there can be no shadow. The light magic and the shadow magic are the same magic, even if they don't realize it. They are flip sides of the same coin.
The void strips the power from both of them. Void is the absence of light.
Of course in the best traditions of YA fiction, light and shadow will combine their strengths to defeat Void.
There are a lot of stuff that can beat them
Light is movement of photons in a specific wave length, darkness/shadow is the absence of light, so they're the other side of the same coin. You can shoot a laser to me, but it will take some time to move from you to me (light is the fastest thing in the universe, but even then it takes 8 minutes for the light of the sun to reach us, so it's not instantaneous).
Time is a movement frame. If i stop time your laser will never reach, if i turn it back it never came out. if i stop time, grab you, put you where the laser was targeting, and put you in an eternal loop of 2 seconds (enough to feel the pain) you'll end wishing you never even meet me.
But it doesn't even have to be something as grandiose as time, a really good telekinesis user can stop movement in the same way, or if it was having a bad day, stop all movement from the molecules of your body and watch how you disintegrate, like when you put something in liquid nitrogen and then hit it against the table.
Heck, even a good mirror is enough to give you some troubles like that.
The bottom line is that every power can become a great weapon or a great failure, if you're creative enough.
Either a mix of both or matter/energy.
It is said that light and dark are two sides of a single coin (in our world one is the presence of light, the other is the absence of it), so one who controls both can easily combine them into something better than each one separately.
On another point of view, you could use an extension of matter control, meaning turning matter into energy and vice versa. This isn't absent in stars, as the light from our sun and any other stars usually come from fusion reactions (our sun fuses hydrogen into helium). That means you can control matter, turn it into energy, mold it into puppets, compress it, fuse it into reactions, cause explosions hotter than our sun's core (see tsar bomba), etc. Making clustering enough matter is also a frightening thing (condense enough matter into a dense enough cluster and you'll get a self sustaining system with such a gravitational pull that nothing, not even light, can escape, aka a black hole)