A big thanks to Woolie Gaming (the king of RoR2) for this info.

1. Right away, look for a Newt Altar. Finding one will get you a blue portal that enables you to get to the Lunar Bazaar.
   • If you're attempting around when the Hidden Realms patch drops, you can find one in the Prismatic Trial near the spawn.
2. Once inside the blue portal, turn around and jump off the cliff
3. There will be an opening in the walls surrounded by a yellow mist
4. Enter, and follow the tunnel to the very bottom
5. You'll see a big pink portal. This will take you to the void realm.
6. Inside, youll have to complete a series of 9 monster waves which start every time you activate a cell.
7. Once you complete the last one, you'll unlock acrid! Congrats!

Notes

• You can activate multiple cells at a time, however difficulty increases drastically. Monsters gain items over time… So be sure you're ready, if you're going to attempt this.
• After you complete all the cells you can still return and keep doing the wave challenge once per run, and you'll get a red item instead of Acrid.