The Morden Towers - Act 1 Story Quest
The Morden Towers is given to you by Warden Celia after you complete the Prisoner of War Story Quest. For this quest you will need to travel throughout the Northern Greilyn Jungle and destroy the 4 Morden Towers that are placed along the path and blocking your route. Each of these towers is marked on your map with a golden star and placed along the linear path so they're difficult to miss.
In order to destroy these Morden Towers you'll need to first destroy one of the cages that are right next to each of the Morden Towers and loot the Cage Fragment from the ground. You technically only need to do this once because you can keep the same Cage Fragment for the entire quest. Equip the Cage Fragment and then left click on the nearby campfire to light it - then left click on the tower to destroy it.
It won't take you very long to find a Lectern which are little golden podiums that you come across throughout the world that teach your character Chants. Two of the easiest Lecterns to find early game are the ones next to the Northern Greilyn Jungle Teleporter, a tad bit west of the Teleporter you'll find the two Lecterns on a ledge. Chants are used at the Incantation Shrines to give your characters short term buffs that are very powerful.
While traveling through Northern Greilyn Jungle you'll also run into Morain who will give you another Side Quest, The Hak'u. I strongly recommend you complete this Side Quest as well as the follow up The Hak'u (Part 2) because the follow up will reward you with an additional Skill Point. Only a few quests in Dungeon Siege 2 offer Skill Points as a reward and they're ones you definitely do not want to skip.
Tip: Gold stars on your map mark Story Quest objectives that you can complete. Silver stars mark Side Quest objectives that you can complete.
Available Side Quests:
Morain fighting a Hak'u in Northern Greilyn Jungle.
All in all the path through Northern Greilyn Jungle is quite linear. I recommend that you explore everything and get into the habit of doing so, you will find many secrets this way and many Side Quests. Dungeon Siege 2 has many hidden chests and rooms which can be opened with things like nearby buttons or levers that are somewhat hidden (like the pictures below).
I recommend you get into the habit of mousing over things to see if they highlight when you do. If something highlights when you mouse over it then it means it's clickable and will often lead to some sort of secret room or area. Once you play the game long enough you'll get the hang of figuring out where secrets are hidden; you can often see the outline of hidden doors in the wall if you look closely enough.
You should also squint at your map and look for blue dots on it. A lot of the time the map will have a blue dot on it where there is a button/lever/secret you can discover (or piece of loot you've left behind), it's insanely small and hard to see but it's there. It took me over 80 hours of playing this game to even realize the blue dots existed and what they were for.
A 'hidden' button which you can press to control the nearby elevator.
Hidden Lever inside of the Hak'u Caves.
After you've destroyed all four Morden Towers you'll want to return to Warden Celia in Eirulan and speak with her. She'll tell you that your collar can come off and she'll send you to speak with Taar in the Great Hall. Taar will remove your collar and give you your stuff back. You'll also be diagnosed with the plague during this time which will begin the next story quest, The Plague.