Olympus - Act 3/Act 4 Story Quest
Olympus is the final quest of Act 3 and was the final quest of Titan Quest until Immortal Throne was released. You begin Olympus immediately after going through the portal that opens up after completing the previous quest, Under Wusao Mountain. Next to you when you enter Olympus is a Portal, incase you ever need or want to come back here.
The path throughout Olympus is filled with some of the most difficult enemies in the game. You will face Gilded Dead skeletons who can dispell all of your buffs as well as Limos enemies which do a ton of Vitality damage and drain your life. Closer to Typhon you'll find yourself against cyclops enemies which are the hardest of them all - their yell stuns you and disrupts your skills. You'll typically be fighting a half dozen Cyclops at a time too which will leave you in a permanent state of not being able to use any of your skills unless you are careful.
All in all, Olympus is a very difficult area to traverse through and Typhon, the final boss, is no easier.
Typhon is the final boss of the original game and probably still the hardest boss in the entire game. He has multiple abilities that you need to learn and at least two one shot mechanics. Meteor Shower and Thorns will kill you if you don't pay attention even if you are twinked out the ying yang in excellent gear.
For this boss fight you will need to have high Vitality, Fire, Lightning and Poison Resistance. Energy Leech Resistance will help too but isn't as important. Out of all the listed resistances, Vitality is arguably the most important, Typhon's Health Leech attack is Vitality based - Life Leech Resistance allegedly doesn't work for it. Below are a list of every attack Typhon uses and a little bit of information about the attack.
Tip: Do not use pets on this fight, Typhon will Life Leech them for the full amount and completely heal
Poison Orb: Typhon chucks a Poison orb in front of him that explodes on impact. He'll follow this attack up with an Energy Leech.
Energy Leech: Typhon does a breath animation that leech's energy from you and is blue. The closer you are to him when he does this the more Energy he'll leech from you.
Health Leech: Much like the Energy Leech, this is a red breath attack that steals your life instead. He only does these leech attacks when you are close to him.
Fire Breath: Standard fire breath attack that deals fire damage.
Meteor Shower: Probably the most deadly attack Typhon does or top 2. There is an audio trigger letting you know that this attack has begun, listen for it then move before they hit you. Meteors will most likely kill you if you do not dodge them.
Thorns: Typhon will coat his entire body in thorns for a few seconds. You'll kill yourself if you deal Physical Damage and you attack him during this time.
Lightning Ball: He'll toss a ball of Lightning in front of him which deals Lightning Damage.
Melee will have the hardest time during this fight since all of his most deadly abilities can harm you. Casters and Ranged will not have to worry about Thorns since they will not harm you. Pay attention for the audio queue involved in Typhon's Meteor Shower ability, they will always come crashing down where you're standing when it happens. So you only have to move out of that spot.
Surrounding the arena that Typhon is in you'll be able to find multiple shrines, all of which will do something different. Collect the important ones throughout the battle that will help you, like a Shrine of Mastery and pop potions like a mad man. That's the best advice I can give you for this encounter.
Once you defeat Typhon, if you own the Immortal Throne expansion and have it installed a portal will open up that you can go through. Your quest 'Olympus' will also complete. The portal will take you to the City of Rhodes which is the first town of Act 4. Speak with the ghost in front of you to begin the first story quest of Act 4, A Mysterious Message.