Gears 5
181 Achievements
2,500
1000+h
PC
Xbox One
Xbox Series
Chamber of Horrors
Completed a match of Horde Frenzy on Atrium
5
1.2%
How to unlock the Chamber of Horrors achievement in Gears 5 - Definitive Guide
ExoticMisfit
TrueAchievements
TrueAchievements
Go to Horde then select custom match and start a private match on horde frenzy with the map Atrium with bots and set to wave 12. After wave 12 is completed the achievement should unlock and you get the 5Gs. 
P.S. Its also worth mentioning you can do this for all the maps for the game to do them through custom to get the achievements for multiplayer, horde, and horde frenzy.
P.S. Its also worth mentioning you can do this for all the maps for the game to do them through custom to get the achievements for multiplayer, horde, and horde frenzy.