The Crimson Rift is a system where monsters appear at certain times. 

Every day at 5:00(UTC) and 14:00(UTC), the Crimson Rift is randomly activated in three areas of the Nuia continent.

You'll be able to see their locations 3 minutes before the start,

and the Monster Level of each area determines the Level of the monsters that will appear. 

When the Crimson Rift occurs in the Solzreed Peninsula, Dewstone Plains, or Gweonid Forest, it will progress up to Rank 3,

and elsewhere up to Rank 4, with each rank lasting up to 10 minutes. 

If you fail to defeat the monsters during the duration, they will disappear and the Crimson Rift will end.