Purchase Shadowy Potions from Gorzeeki in the Burning Steppes. Use the potions to travel through Jaedenar, and speak with Lord Banehollow.
Mor'zul says that you want to open a portal to Xoroth. To do that, you'll need Xorothian stardust. That's not easy to find on Azeroth!
There is one being who might have some: Lord Banehollow in Jaedenar, the seat of the Shadow Council! Speak with him and he might part with the stardust.
But if you're going to Jaedenar, then you'll need to cloak yourself in the Shadow Council's own flavor of evil. That will require some of my shadowy potions, and that will cost you...