A part or stage in a game that escalates in difficulty in a very short period of time, separating the casuals from the actual gamers.
Not to be confused with a steep/vertical learning curve, as these
mean that the game is hard to learn and master from the beginning.
Games with a
casual filter will give you a false feeling of confidence as the filter comes after you have learned the basics and started progressing, just to catch you off guard and make your run end, usually having to start from zero again.
Some examples of
casual filters in games include the
winter in Project Zomboid and your first goblin raid in Dwarf Fortress
Person 1: Ayo wtf is this I just
lost my 30h run, how do you even survive the
winter?
Person 2: Welcome to the
casual filter, play more and git gud