The Demonwell was a well sought out by Gimbolt Surehammer, Maer-thiss and Ziggler.

Speculation

This article or section includes speculation, observations or opinions possibly supported by lore or by Blizzard officials. It should not be taken as representing official lore.

Judging by the name, and Maer'thiss' anger over Ziggler interrupting the ritual (the ritual they needed to find the well), it possible that it is the Chaos Well created by Mannoroth. The chaos well that Grom Hellscream and the orcs drank of before they became chaos orcs and killed Cenarius.