Guttermouth Rewards refers to the Guttermouth Cells that can be found in certain places of the Nightmare. Similar to the Otherworld Tower dungeon, the Guttermouth Cell Challenges are mini-dungeon instances that players can enter with the usage of a Guttermouth Key to fight against various monsters in order to acquire rewards, both familiar and unfamiliar items.
Guttermouth Keys are rare drops that can be obtained from slaying monsters in the Nightmare. Once the player gets their hands on a Guttermouth Key, they can use the key to enter one of the four available Guttermouth Cell Challenges. These keys can only be used once and will be destroyed and disappear from the player's inventories after being used to unlock a Guttermouth Cell.
There are three Guttermouth Cells and one Guttermouth key-locked door in the Nightmare. Two of which requires the players to obtain a Mansion Key beforehand. The Mansion Key must be in the players' inventories in order to find two of the three Guttermouth Cells. These cells are located to the left and right of The Vault's entrance in The Grand Hall, under a short series of stairways leading into different rooms, both locked and unlocked. The other Guttermouth Cell outside of The Grand Hall can be found to the right before entering 'The Library' for the first time in The Mansion. Before entering 'The Library', the players will need to head to their left which appears to be an empty hallway. Down the hallway, the players need to take a right to find an obscured hallway on the sides of the wall. As the players travel through a series of rooms, they can find a bedroom with stairsteps to direct the players to the second floor, leading into an attic with a Guttermouth Cell.
Each Guttermouth Cell provides a different scenario that players must go through in order to complete the Guttermouth Cell Challenges. Defeating the boss within and its minions/allies will reward the players with a treasure chest. Information pertaining the rewards can be found later on in this article.
Cell 1 & 2
Going to the Guttermouth Cell to the left of The Vault's entrance in The Grand Hall will require the players to defeat two Red Wolves whilst having to fight off an Industry Giant as the main threat of the challenge. Going to the Guttermouth Cell to the right of The Vault's entrance in The Grand Hall will require the players to fight against a pair of Crickets (Theatre boss), as well as a Nightmare Lantern.
If the players decide to head left down the hallway before they enter 'The Library' in The Mansion, travel through a small obscured hallway within the side of a wall, and travel up the stairs in a two-floor bedroom, the players will come across the third Guttermouth Cell. This is considered one of the most difficult fights of the three Guttermouth Cells, requiring the players to fight against Two-Eyed Monstrosities and a Guttermouth Effigy, the main threat of the cell challenge that needs to be the top priority in the kill-list while fighting against the mobs in The Lost, as its ridiculous speeds and healing ability can be annoying to the players.
The Lost's Boss Fight
The room can be found near the giant Ratboy Statue in The Grand Hall, located to the right/left depending on the player's position. It has black chains on the door, like other key-locked doors seen in the Nightmare. Once the players enter the door, they will face against The Lost's boss, sharing the same exact title of the map area. Players will fend off against the phantom, nearly-invisible minions of The Lost, as well as the boss itself.
Below are some of the rewards the player can receive from completing any of the Guttermouth Cell Challenges offered in the Nightmare after the players achieved victory over the monsters they had defeated in The Lost. If you would like to see more information about percentage chances, please use the public list created for your own leisure! --> https://pastebin.com/j2C9sdWq
General rewards simply mean that the rewards can be obtained from any Guttermouth Cell Challenges. Here is a list of rewards that players can receive from the treasure chest after completing a Guttermouth Cell Challenge: