@endlessdivinities has liked for a starter.
The Void - always called a space or referred to as a place.
But Gaster didn’t believe it was either of those things; the interpretation however made it easier to imagine though.
No, the Void was more of a concept, a concept that rejected the law of the physical and made it inconsequential - a concept he came across trying to pierce the veil between his world and the next.
Time. Space. Proximity. Form. All referred to the physical - and all very meaningless here. In this boundary between the physical and spiritual, he existed everywhere and nowhere, immortal yet dead, and connected and isolated all at once.
Because this was the Void where physical law had no say.
In here, he simply was. He thought, therefore he existed. Nothing more. Just an eternal observer looking down upon his metaphysical throne. And it was from this position that Gaster spoke.
No words would be said, for words would require a body and a medium to pass them through, yet the message would be conveyed as if he were communicating with this new stranger soul-to-soul.
“...Something comes. A cog in the gears of how your world works. A thought and many thoughts not your own shining through your SOUL.”
“Fascinating. Humor me, little passing divinity. How have you ended up here?”