The Underworld node is the one everybody asks about. It appears on the skull in the middle of the room, but only once all five braziers are burning at the same time.
Where it is
The Underworld. The marked map is on the location page: The Underworld.
Getting it
Each brazier is tied to one of the town portals — Tos, Cornoth, Barloque, Marion and Jasper — and lighting one flips two others out as well. Tos flips Tos, Barloque and Marion. Cornoth flips Cornoth, Marion and Jasper. Barloque flips Barloque, Jasper and Tos. Marion flips Marion, Tos and Cornoth. Jasper flips Jasper, Cornoth and Barloque. Clicking a brazier that is already burning does nothing, so you can only ever light a dark one, and you have to stand next to it.
From all five dark, the shortest run is seven lights: Tos, Cornoth, Marion, Tos, Barloque, Tos, Jasper. The fifth flame brings the node up on the skull at the centre of the room, and every town portal is open at that point too.
Melding
Stand within three squares of the node and use it. The bond raises your maximum mana by three points plus one for every ten points of Mysticism above five, so between three and eight depending on how mystical you are. You can bond with each node once, and the game will list the ones you hold if you ask.