So let me try to help you with this problem.
So what is World ending? Well it looks something like this:
View attachment 100928
I marked it with the black lines.
For Art terrain you dont want to leave it there, cause it will make your pic look unrealistic in bad way, cause have you ever seen world end? I thoughted so. xD So to heal that problem there are many ways.
One most used way is like you said, making mountains in background. This will hide world end and make your picture look better. Something like this:
View attachment 100929
Other way what you also use next: You need to use fog and bring it close enaugh close so it would hide world end. Heres a picture, so you could understand better what i mean.
View attachment 100930
Ofc there are many, many more ways to hide it: Use props, change camera angle bigger, make forest etc.
I hope this helped you.
P.S Dont look my exsample pic as art terrain cause they were made fast to illustrate my ideas.