Here is the full list of all 17 Red Obsidian Remnant achievements.
Kill at least three adventurers in an adventure.
The first clearance of the 『Underground Prison』on the 3rd floor will unlock『The Corruption』on the 4th floor.
Successfully passing trials of four floors under the 『Easy』 mode.
The clearance of the『Hard』mode will unlock the『Nightmare』mode.
The clearance of the『Nightmare』mode will unlock the『Limit』mode.
Successful completion of six challenges in an adventure will unlock the 『Super Challenge Room』.
Hire at least three adventurers in an adventure.
The clearance of『The Corruption』under『Nightmare』mode will unlock『Devil's Gift』.
Successfully passing trials of four floors under the 『Normal』 mode.
Successfully passing trials of four floors under the 『Limit』 mode.
Clearance the『Limit』 in multiplayer mode