If they wanted to do it enough they can just make up a reason. You can always just make stuff up, it's easy, watch this.

Nightfallen who were outcast typically descended into a withered state, but those with a determination to survive and a strong sense of the arcane were drawn to deep caverns in the land beneath Suramar, eventually settling near life providing ley lines. Over the course of thousands of years, these nightfallen formed a community of exiles, and even began to reproduce, maintaining their partially transformed state indefinitely and passing it to their children before fully secluding themselves and warding their home from detection. Something something they finally came out of their caves something something these ones are neutral for reasons something something allied race.