BC6 Marsh Rooms is an unlockable challenge in Benighted Soul, a mod for The Binding of Isaac.
This challenge is played on Hard mode.

Yum Heart
The Battery
PHD
Blood Bag (x3)
Old Bandage
Little Baggy
Car Battery
Spirit Sword
Birthright
4.5 Volt
Candy Heart
Sumptorium (Consumable slot)
NO! (Smelted)
Red Hook
Circumcision
Golden Key

Note: this list does not include characters' default starting items.

Marsh Rooms

The following is unlocked for completing the BC6 Marsh Rooms challenge: Benighted Eve starts with another Eve Baby.

Unlock Benighted Eve.

• Download Benighted Soul from the Steam Workshop.