It is of 5000 Shallows enemies, but most of the specific ones are listed below.
-1000 Harvesters of Souls.
-800 Shadow Golems
-998 Chaos Keepers
-1 Harvenger of Death, and 1 Eye of the Bone Catalyst(at the same time). The two bosses ALSO appear at a random time.
This will bestow upon you Shallows monsters drops during the event, and only certain items that can be earned from this event, such as the Joy Eraser, or World Tearer. You are also given a free Nihtrachi and 300 Nexus Arrows.