CANNOT FINISH THIS TASK

This task want me to clear the abyss in the North.
But I'm stucked here, for I can't finish this map.
As you can see, I have killed the boss, but this map was not finished. It shows that I still have to "clear the abyss" in this map.
Problem is here: there is no abyss in this map.
I thought it might be an usual bug, so I quit and replayed this map for three time, it doesn't work.
Meanwhile, as it is a "tried" map, I can't add abyss to it manually.

D:/Desktop/1/2026060101.png
D:/Desktop/1/2026060102.png
D:/Desktop/1/2026060103.png

Omg this chinese doesn't know how to upload images :|