Leaping into the whirlpool successfully plays a cinematic of the player caught in the whirlpool as you fall into the Bashal'Aran Collapse. The night elf Bielara Ivyshroud sits next to the pool at the bottom of the Maw, and at the base of a slope. She asks you if you wish to keep her company or return to the surface.
At the top of the slope is the shade of Telarius Voidstrider <Herald of the Dark Embrace>, killed by Maiev Shadowsong a long time ago. He will attack on proximity, and transforms into a large demonic appearance during the fight.
When Telarius is defeated, he drops the scroll, Writings of the Dark Herald, which provides a quest to return it to Ranger Glynda Nal'shea back in Lor'Danel. The scroll itself purports to be tutorials on becoming a demon-hunter. Bringing the scroll to the ranger provides the Ghosts in the Dark achievment.
Forlorn Highborne will appear on the slope as you return to Bielara. She will teleport you to a shallow cave immediately south of and overlooking the Maw.
Notes
This fall may also provide the Going Down? achievement.
Swimming into the Maw only provides an less than interesting view of your character's kicking feet. It is possible to reach the "bottom" of the Maw and back to the water's surface before running out of breath. The cutscene-quest sequence is only achieved by jumping from height.