The Kalrathian Nexus - Act 3 Side Quest
Closest Portal: N/A
Objective: Charge the Soul Staff at 3 caverns then return to Kalrathian Nexus and smash the seal
Reward: Soul Staff of the Nexus + Random Loot & Gold
The Kalrathian Nexus is a Side Quest which begins by speaking with Nora the Nature Mage in the town of Kalrathia (Act 3). When you accept this quest Nora will give you a Soul Staff as well as a Mysterious Chant (Chant 76). Both of these items are required to complete the quest, so you'll want to keep track of the staff and make sure that you do not lose it.
For this quest what you need to do is travel to 3 nodes in different areas of the game and equip the staff on one of your characters while reciting the chant that Nora gave you (Chant 76; the final one in your book). Once you've done this at all 3 locations you'll need to take the staff to the Kalrathian Nexus in the Eastern Plain of Tears and use it to destroy the seal of dark magic there.
Below I have included a succinct list of where you can find each of these 3 nodes and a little bit of information about each one. If you need additional guidance in finding any of these locations or finding the Kalrathian Nexus check further down the page for more details and screenshots.
Cavern of Earth: Temple of Xeria; go west from the Teleporter
Cavern of Frost: Snowbrook Haven Servants' Quarters; go west from the Snowbrook Haven Living Quarters Teleporter and up the elevator
Cavern of the Phoenix: Lost Valley of the Azunites; go directly south from the Teleporter until you reach the cavern
Cavern of Earth in the Temple of Xeria.
Cavern of Frost in the Snowbrook Haven Living Quarters.
Cavern of the Phoenix in Lost Valley of the Azunites.
The Kalrathian Nexus in Eastern Plain of Tears.
Once you've destroyed the seal in The Kalrathian Nexus all that's left is to return to Nora the Nature Mage in Kalrathia and speak with here to complete the quest.