Voidsea

'One of the earliest records we have of the Voidsea refer to it as 'Outer Space'. As far as names go, it's appropriate, I suppose - They thought of 'Space' as being 'outside' their worlds, but the truth is the opposite - We are always within it. Our worlds are but islands within the Voidsea.' - Captain Abyss-Breach, 17th Century of the Valhallan Age

Everything is within the Voidsea.

The Voidsea is the endless expanse that makes up the known universe, and contains everything within it. Void Islands, Stars, phenomenon - All of these things are contained within the Voidsea.

It has been known by many names in the past - Outer Space, the Black Sea, the Ether, the Void

It's defining feature is the vast, uninhabitable distances between Void Islands, Void Coves, and other celestial bodies, requiring the use of advanced technology, biological engineering, or other such adaptations for many lifeforms to travel between worlds while in this reality. In contrast to it's counterpart, the Realmlands, this expanse is often compared to an endless ocean of nothing, hence the name, Voidsea.

As a matter of understanding, the Voidsea refers to the entire reality itself - anything within it is a part of it - The Sky River and all it's satellite Gyres, the Void Islands, Shoals, and Void Archipelago's are all part of the Voidsea.