How to unlock the newest survivor in Risk of Rain 2, Acrid. He is a new poison doggo that came out with the recent hidden realms release. To unlock Acrid, you have to complete the challenge To Be Left Alone, which requires you to complete the void fields challenge. You have to stabilize the Cell in the Void Fields. This is a wave-based challenge that requires you to charge the null fields like you would a teleporter against waves of enemies with their own sets of items. Quite a difficult achievement to get.