First, I thought the DO curse was darkness to end all darkness. Secondly, he has now all the power of the previous DOs plus his own (i.e. he is now the Ultimate! Dark One), so why would he not be able to go there?
I thought the same, but I guess that just because he has dark powers doesn’t mean that he would want to visit a Hellish place? It doesn’t exactly sound like it’s full of fun.