Restore Kalrathia's Water - Act 3 Story Quest
Restore Kalrathia's Water is the first Story Quest you'll be given in Act 3 and it starts by speaking with the Lord of Kalrathia who is found in the Town Hall. You'll be asked to restore the water supply to Kalrathia and to do so you must travel to the Water Chapel which is found at the northern most portion of the Northern Plain of Tears.
Prior to leaving Kalrathia and starting on this quest I recommend you do some exploring and pick up all of the currently available Side Quests.
Available Side Quests:
Dwarven Song of Ore
The Kalrathian Nexus
The Legendary Mace of Agarrus
The Mage's Apprentice
Mythrilhorn (Find Khartos the Wise)
Mark of the Assassin (Quest update here)
The Lore of Aranna
The Lost Jewels of Soranith
When you're ready to continue with this quest head out of the northern gate of Kalrathia into the Northern Plain of Tears. You'll find our next destination at the northern most end of this area but there will be 3 Side Quests that you can complete along the way. The Lost Jewels of Soranith, Deru's Treasure Hunt and Mythrilhorn are all done in this area.
Inside the Water Chapel you'll have to fight against a Ganth (pictured below) which is one of the more annoying enemies in the game because he constantly heals himself with melee attacks.
To defeat the Ganth the best advice I can give you is to spam your Powers and try to get him to face only one character (your tank). If he keeps healing for all of the damage you deal to him then unequip some Autocast heal spells and focus on Potions to heal. If he still out heals your damage then stop using Summoned Pets and start switching to better Powers; things that deal Single-target damage rather than AoE.... If he is still out healing you then go do some Side Quests and level up more.
Once the Ganth is dead head upstairs and prepare for another difficult fight, this time against a bunch of Morden as well as an elite Morden. Whereas the Ganth was more of a DPS test to make sure you could deal good damage - this fight is a good ol' survivability fight.
Press the Button on the wall when the fight is over to restore Kalrathia's water then return to Lord Kalrathia and speak with him to complete this quest and start the next, The Morden Chief.