It's not often I tell people to go jump in a lake. But you're going to want to go jump in a lake for this one. Specifically, the Maw of the Void -- a swirling vortex of water unearthed with the Cataclysm. Upon discarding all sense of self-preservation and leaping forthwith into the water, you'll be treated to a short cutscene as your character is abruptly sucked under the water, the screen doing dark.
And that's when things get really weird.
At the top of the ramp is another angry spirit -- this time, the ghost of a demon hunter named Telarius Voidstrider. He doesn't waste any time at all trying to kill you. After successfully defeating Telarius, loot his body for the Writings of the Dark Herald, a book of tips and secrets on becoming a Demon Hunter. These kind of secrets aren't exactly embraced by open arms in night elf society, so take the book over to Ranger Glynda Nal'Shea over at nearby Lor'daenel. You can speak to Bielara to get out of the hole.
Turning in the quest will net you the achievement Ghosts in the Dark, and a little history on Telarius as well. It turns out Telarius was a demon hunter killed by Maeiv Shadowsong, long ago. The exact details of their altercation are left unsaid, but the book is taken by Glynda. Although obscure, unearthing this achievement is pretty easy if you know where to look -- and it'll net you 10 achievement points for very little effort. And while there is little to be gleaned in regards to Telarius and his mysterious fate in the depths of Darkshore, perhaps some secrets are best left a secret, and questions left unanswered.