The Sunder are gigantic Dragons that have acclimated themselves to the Void. Due to having so much room to grow, they can grow to be the size of realms. All Sunder use Astral Magic, or other variants of it.
Since there is little substance in the Void, sound doesn't travel well. Instead, Sunder use infrared waves to communicate with each other, using special sensor glands all over their body, and more so on their face.
Spatial Sunder are the smallest variant. Despite being the smallest, they're still anywhere from six kilometers long to 500 kilometers long. These Sunder are highly aggressive, and often attempt to eat each other. Spatial Sunder can't do much to realms, as they're still somewhat small.
Stardust Sunder are larger than the Spatial Sunder, and their control over Astral Magic and their variants are highly tuned. Stardust Sunder are anywhere from 500 kilometers to 5000 kilometers, significantly larger than the Spatial variety. They're large enough to host pocket dimensions on their backs, which they often do. Stardust Sunder are somewhat docile, while they still might attack other smaller Sunder. They're large enough to attack an undefended realm, but if it has an intact bubble or it has some sort of God guarding it, they won't even try.
Cosmic Sunder are the largest known variant, and they know the most advanced version of Astral Magic: Celestial Body Magic. Cosmic Sunder are incredibly large, anywhere from 5000 kilometers all the way up to 25000 kilometers. They're a serious threat to realms, as they can pierce straight through a realm's protective barrier. Gods often have trouble fighting them, when they do show up, as they're incredibly powerful. Being so large, they also often either carry, or guard a realm as their own property, rarely dealing with its inhabitants.