THE ROAD TO SUNSET ISLE
SEYDA NEEN
To start your adventure in The Black Mill, go to Seyda Neen and talk to Gandor Amsirva, who's standing together with two ordinators near the silt strider port. He'll ask you to go to Lionhead, who lives in a shack next to the lighthouse.
Lionhead will tell you about a new hero, and he needs you to get some papers from a chest inside the Census and Excise Warehouse. It's possible to just break into the place of course, but it may be easier (and more fun) to go through the wine cellar beneath the Census and Excise Office. Lionhead gives you a key for that.
Of course you remember the room where you picked up your first possessions. Go right from there and into the basement; enter the wine cellar through the trapdoor.
On the other end of the cellar (don't trip over the mice!) there's another trapdoor, but you'll find it firmly locked. Talk to the guard BalOn, he'll invite you to a game of riddles. A child could answer them really, but in case you forgot how to think like a child, the answers... First, don't worry about getting the wrong answer, you'll only get drunk. And there's a Chasnam Cure Hangover on the table to get rid of the side effects, so have another glass! 
The answers are: fire, water lily, and the wind. I told you it was easy. 
It will become much harder later on... 
Anyway, when you answer correctly, BalOn will hand you the key to the trapdoor. Go up the stairs to the top of the Warehouse tower, and open the locked chest there, which contains the paper you need. Take it to Lionhead. He'll decode the paper and hand it back, so you can take his findings to Gandor Amsirva.
EBONHEART
Gandor needs you to find a certain Hasib (the new hero), and a prisoner called Betelgeuze knows more about his whereabouts. Travel to Ebonheart and locate the prison cell (near the smith Sirollus Saccus) where Betelgeuze is being kept. If you're not very likeable you might need to persuade him to even talk to you about anything at all.
Betelgeuze wants to escape, of course, and to make that happen you need to find his friend Myth. Myth is a soldier in the Imperial Guard Garrison, behind the Imperial Commission room (take the door to the left) in the far end of the castle. Myth should be standing in the far right corner when you enter. After your exchange with him you should return to Betelgeuze, and he tells you to seek Sir Amra in Caldera. He zaps away, but it may not be the last you've seen of him...
CALDERA
Sir Amra is in the Governor's Palace. Go up the stairs in the far left corner, to Odral Helvi's bedroom two flights up. Talk to Sir Amra, and be honest! He'll direct you to the Caldera Mines to find Hasib.
Sir Amra Darius
After passing the Mining Company Office you'll see a signpost pointing right towards the Old Mines; maybe you can see the Ebony Mill in the distance. Be careful when you enter the mines, as you always should be when you go into caves and ruins and the like.
The entrance to the Ebony Mill itself is up the stairs behind that first miner you encounter, but you won't get very far in there, you need a key first. So continue into the mine passages to find Hasib, as Sir Amra advised. And mind your head!
You might get confused about where to go in there; it's easiest to just follow the railway track, although there's nothing against exploring elsewhere and confront the baddies that hang around there. You can also sneak past, but you can't avoid all of them...
Keep following the track to the end, and find out another reason to be afraid in the dark. Crawl through the hole into the rearmost section of the mines.
Now's the time to use a Rising Force potion. Go up and to the right to another railway track; to the left you'll see an old railcart. A little further the track is blocked by a grate, but pushing the railcart will get you through. Then there's another grate blocking your way, and no railcart you can use! But you may remember from what Sir Amra told you, that those railcarts and mechanisms are driven by the mill; there's a cave with buttons to operate everything, but you need to crawl back through the black hole to find it (if you haven't already).
Walk from there to a passage on the right that leads up to a lava stream, and a cave behind that. In that cave are buttons inside alcoves, and a row of bells, which can give you a clue about which button to push. Or just take your best guess and take the damage, of course. 
(It's the leftmost button.) Then go and crawl through the black hole again.
Take it from where you left off, and follow the railway track to the end. On the far end you'll meet Hasib; talk to him, and he'll give you the key that gives access to the upper part of the Ebony Mill. Pay attention to what he tells you, and RUN!
Find the stairs leading up to the mill entrance (back in the front section of the mines) and proceed upwards. You'll need to levitate again for this... Then defeat JurGor, and pick up the book Terra.
Return to Sir Amra with the book. When you enter the Palace you'll hear the sounds of fighting - Sir Amra is being attacked! You can rush up the stairs if you want to save him, but it'll be futile, he will die anyway, sadly. Read your journal to find out what to do next (as you may have made a habit of by now.)
HLA OAD
Seek out Gandor Amsirva in Seyda Neen again, he'll know what to do: visit Tommy! You can walk through the mountains, or take the silt strider and boat to Hla Oad together, it doesn't matter. If you walk, Tommy's shack is the first one you'll see, to the left; stepping off the boat, you'll see it on your right. Enter the shack and talk to Tommy, to find out more about what's going on.
He'll ask you to investigate the windmill in Hla Oad. Inside there's a trapdoor to a cave complex. Kill the orcs down there, and search the orc Captain - he has a key. Go left through the wooden gate, and eventually you'll see a locked door to use that key on. Look inside a crate, and you'll get a journal update. Then return to Tommy.
After another talk, Tommy suggests that you should go to Sunset Isle to find Lady Stadi Galanos in the Cathedral there. Exit the shack and walk to the left (straight to the west). Past the ship that's anchored there, you may see some rocks; that's where Parakiet lives, in a cave. Parakiet will teleport you to a ship.
Talk to the sailor CopyD about travelling to Sunset Isle, and he'll direct you to the Captain's cabin. The Captain, Cyclod, will then take you to the isle.
SUNSET ISLE
Exit the cabin and look around: you're finally there! An unknown island for you to explore. Visit the seedy bars, enjoy the generous hospitality of the Smoking Pipe (but keep your hands off the waitresses!) and get some sleep there, check out the shops and market stalls, or proceed directly to the Cathedral - it's your choice. You may even be distracted by some side quests and forget about the main quest for a while; I'll tell you about those later. Walk around the harbor to the other side, and you'll soon see (and hear) the Cathedral.
To be continued on Sunset Isle, or in the Cathedral.