A 5-second jumpscare chamber, not an explorable place.
Dark Void is not a separate explorable place, it is a 5-second jumpscare chamber. Spotting or approaching the Lost Player entity, within 16 blocks, in the overworld, while World Corruption is not at Level 2, triggers it.
The player teleports onto a tiny platform, surrounded by 5 Lost Player entities that "flicker" by respawning every 20 ticks (1 second). After 100 ticks (5 seconds), every entity is destroyed and the player teleports back to the exact position and facing direction they started from, earning the "they_walked_in_the_dark" advancement. A 5-day cooldown follows before it can trigger again.