Charleston Nib is an archeologist researching an ancient temple of Kozah in the Archaeological Site area south of Lighthouse. When you first discover the dig site in the center of the area, you will find Charleston and six "diggers" (who look and act more like guards, with spears rather than picks or shovels).The diggers all give you the same advice if you speak to them: 

Psst... 'ere, there. I would be leavin' as soon as ye can, mate. Bossman there is nice an' all, but he have no real idea what be going on. I tell you, there is some bad magic in the air.

When you speak to Charleston, dialog responses 1, 1, 2, 3, 1 (shown below) will get you a job as security for the dig, paying 100 gold for one day's watch on the site. 

Charleston will leave once you are done talking to him, and Gallor will then approach you with a different offer.

Note: If you walk southeast before speaking to Charleston a second time and you have not already encountered Brage, you will be confronted by him. If you correctly answer his riddle and try to help him, as part of the Brage's Life Spared side quest, you will be whisked away to Naskel. You can return and talk to Charleston without interrupting this dialog.
Once you are done with Gallor, you must speak to Charleston again at the entrance to the excavation to advance the story: 

Success! We are about to enter the last remaining room! It's sure to be the shaman's abode!

— Charleston Nib

Then comes a cut scene in which you follow Charleston into and through the excavation to a chamber with a stone sarcophagus, where Charleston's joy at the discovery is quickly marred by all of his workers going mad and attacking your party.  You are then given the choice of concluding your contract peacefully, for whatever gold reward you may have negotiated and 1000 xp, or you can kill him for a huge reputation penalty, 7 xp, a Sunstone Gem and your gold reward (which may be zero gp, 50 gp or 100 gp). 

Finally, you can loot the sarcophagus for the Idol of Kozah (which Charleston has warned you to leave untouched), and carry it outside for a challenging encounter with its guardian Doomsayer.


The dialog that leads to the 100 gp offer is shown here. (Response sequences 3, 2, 2, 3, 1 and 2, 1, 3, 3, 1 also work.)

Note: It appears from the game files that under certain circumstances (probably low Reaction) the dialog options may change, and this sequence may not be available.

Put yer backs to those shovels, men! If she's not out tonight we'll lose 'er to the bandito scum!

You there! State your business, but don't move from where you stand! I don't want to have to sic the boys on you!

1:-Stay your wrath, we are just passing through and mean you no harm!

No? Good then. Sorry for the rough greeting, but it has surely been slow digging with bandits harassing us nightly. Charleston Nib, at your service.

1.1:-Well met, sir. What exactly is your goal here? You look to be excavating a tomb of sorts.

You are partially right, though it's not a tomb, but an ancient settlement. It is also all that remains of a primitive race long since extinct, and we know little about their culture save that they seem to have destroyed themselves in war. Perhaps today we will learn something more. We are just about to break through into an inner chamber, perhaps the dwelling of the village shaman himself. Mayhaps you could be of assistance to us? We need but keep the camp secure a short while longer. Your very presence would probably dissuade anyone from attacking us, though I do not know the force behind our misfortune. They seem to take great pleasure in minor sabotage and kidnapping, though some of the missing men may have just run off from fear. Nothing is ever stolen, which is odd for bandits, but they must still be the cause. Will you help us?

1.1.2:-What would be in it for me? Knowledge is well and good, but I have bills to pay. 

We have little to offer, but would 50 gold suffice? It's all we can give, what with having to restock our camp thrice over. Damnable saboteurs! Your mere presence may be enough to discourage our mystery assailants. Are ye up for it? It would only be for a short while. paltry coin is not worth my time! Raise your offer or I am leaving!

But... as you wish. Perhaps we can scrape another 50 gold together, but it means that we MUST be done today. Is 100 gold total satisfactory? acceptable price for potentially doing nothing. Continue your work and I shall keep watch.

Gracious we are! Now we stand a chance of completing the most exciting day of our dig! We're going to try for an entire new room today, possibly the shaman or chieftain quarters! We could find any number of relics within. Move amongst the men as you will, as your presence will no doubt reassure them.