Zaramoth's Horns - Act 3 Story Quest
Zaramoth's Horns is the penultimate Story Quest in Dungeon Siege 2 and begins immediately after you complete The Agallan Giants Story Quest. Really all you need to do for this Story Quest is to climb to the top of this dungeon. It's a long and linear route - you may get confused and turned around a few times (I did) but that's ok because the extra EXP can't hurt.
As far as leveling up and grinding for gear goes, this dungeon is one of the best places to do it because of the sheer number of enemies that you'll encounter here.
Side Quests In Area:
Spirits of Aranna
Throughout this dungeon there will only be 2 Teleporters to find, one at the bottom nearby where you enter and another right before the Archmage of Valdis boss fight.
The penultimate boss fight of this game will be against the Archmage of Valdis. This boss has a slightly different strategy than the Dark Wizards we've faced off against before and he is also the first boss in this game you're going to fight with a one-shot mechanic.
Instead of summoning crystals that make him invulnerable he'll create objects called Aegis Apocryphal which you'll need to destroy before you resume attacking him. Make sure you are in 'Mirror Mode' when these spawn and you go to take them out.
The Archmage's most deadly attack involves the obelisks around his boss platform. Occasionally he will activate one of the 5 obelisks in the room and they will shoot skulls out towards the center of the arena which move relatively slow. If any of these skulls hit you or any of your pets they will instantly kill you. You have two ways of dealing with this attack.
Obelisks only have about 500 HP (on Mercenary) and if you 'defeat' them they'll stop shooting skulls at you. For this fight you'll have two obvious choices for dealing with the skulls:
1) Use Mirror Mode and dodge all of the skulls that come out at you and to destroy the Aegis Apocryphals that protect the boss
2) Switch to Rampage mode when an obelisk becomes active and use a Ranged attacker to hit the obelisk a few times which will deactivate it
I'd recommend ignoring the obelisks unless your DPS is on the lower side in which case you'll definitely want to destroy the obelisks so you do not get overwhelmed. Aside from this one attack the Archmage doesn't have many other deadly abilities that you need to worry about.
After you've defeated the Archmage of Valdis don't forget to collect your loot from the surrounding area. You will find multiple sets of chests along the upper ring of the arena where this fight takes place as well as the loot that drops from the boss himself.
Defeating the Archmage of Valdis will also complete this Story Quest. Completion of this quest will lead you to the last Story Quest of Dungeon Siege 2, The Final Ascent.