The Temple of Xeria - Act 1 Story Quest
The Temple of Xeria is the 10th and final Story Quest in Act 1 of Dungeon Siege 2 and it begins immediately after you complete the Windstone Fortress. For the first part of this quest you will be tasked with clearing out all of the plagued soldiers inside of the Temple of Xeria. This shouldn't pose too much trouble since most of the soldiers are easy enough to find.
I recommend you take your time when searching the Temple of Xeria, as there are many secrets and Side Quest areas to be found here. A tip I like to give often is squint and look at the map for blue dots when you're hunting for secrets. Blue dots will mean you either left a piece of loot at that location or there is a secret to be found there.
Available Side Quests:
When you reach the room with the glowing blue statue (pictured above) you'll need to clear all the monsters in the area then click on the statue to proceed. If you picked up the Secrets of Xeria's Temple Side Quest from Eirulan earlier then you'll have a silver star in the previous room too with a quest to complete.
All that's left of the Temple of Xeria are a few more enemies, an Incantation Shrine and a Teleporter. Next to the Incantation Shrine is a Statue you can click on that leads to a secret passage. You'll find a Spirit and a room used in the Kalrathian Nexus Side Quest later on.
When you're ready for the final boss fight of Act 1 travel north of the Teleporter until you stumble into the Giant Trilisk.
This is the first 'real' boss fight of the game and in order to beat it you'll need to defeat all 3 of its heads. Switch your party to 'Mirror Mode' and focus all of your attacks on the center head, Vitalus. We want to defeat Vitalus first because he has the ability to heal the other two heads - for a lot.
I'd recommend you focus on Vitalus and don't worry too much about avoiding the AoE attacks that this boss throws out. Spam Potions instead to negate any damage you take and also use your Powers frequently. Nature Mage Invulnerability helps a lot for this part as does any Fighter with Brutal Attack as it'll deal heavy damage to the boss.
Once you've defeated Vitalus the other two heads will be pretty straight forward - just keep spamming Potions and Powers until they all go down. With all 3 heads defeated there will be a short scene after which the boss will be dead. Destroy the crystal behind the boss to trigger another scene and uncover the portal to Aman'lu as well as Act 2 of the game.
Going through the portal will complete this quest and begin the first quest of Act 2, The Town of Aman'lu.