The orc caves node, deep in the cave east of the entrance.
Where it is
A Deep, Dark, Spooky, Icky Cave. The marked map is on the location page: A Deep, Dark, Spooky, Icky Cave.
Getting it
No trick beyond surviving the walk in. The node is always in place.
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.
This node can be drained by the Xeochicatl. See Nodes for what happens when one is attacked.