"I hope you don't mind", says Captain Footbeard, "if we make a little detour on our way."
"What sort of a detour?" "Oh, nothing serious ... but you must have heard the stories of the Tieflow".
The Tieflow is a fearsome name, and there is little Captain Footbeard can say to reassure you. "I can't say as 'twill be easy", he comments, shading his eyes, "but we will be fully armed and ready for the Forest. The worst any of us 'ave 'eard is darkboar, leaping onto boats in the fearsome dark. An' old women speak of darkwolf, with great fangs of power, but no boatsman has ever seen aught like that. Old wives' tales, y'see." He puffs on his pipe, the tobacco burning red in the pale dusk.
"And a'course you'll be adventurers", he says abruptly and laughs. "Pr'aps - pr'aps you'll 'elp me 'stead the other way around!" He finds this unbelievably hilarious, and is soon laughing uproariously.
The captain's humour is understandable: you aren't an especially imposing character, standing before him in your adventuring clothes, your weapons clasped close to you in the chilly late-autumn air. Most of the captain's own crew - standing on board his boat, the Fann Yar - look a whole lot more imposing than you do. You need a boat to take you downriver to the city, but boats are scarce at the moment. Despite childhood stories of the dreadful Forest of Tief, you decide - through desperation, defiance or deliberation - to take the captain up on his offer. Several other adventurers seem to be heading in the same direction, and they come on board as well. About two days after the last significant settlement, the boat travels off the main trade routes onto the Tieflow tributary, heading towards the Forest of Tief. Within the week, the dark forest rises up ahead of your boat and has soon surrounded it utterly.
The journey through the Forest - two long days, the entire crew on alert throughout - go by without incident. On the first day, you hear a curious keening from the very tops of the tall trees that encircle the banks of the river, a ghostly sound from another world. Late on that first night, your crewmembers - but not you - encounter their first ghost, a pale feminine figure who appears first lost and confused, but as her pleadings for a little water to drink go unheaded, she rears up, a terrifying nine-foot tall visage of hate, screeching as she flies over the ship four times and wakes everybody up, including you. By the time you run to the deck, the visage has flown off, and the crew - terrified - are making jokes and whispering obscenities to hide their fear. The second day and night go by without any incidents, and by the afternoon of the third the trees have thinned out, revealing open forests, marshlands along the backs, a large hill - "Tiefstern", one of the boaters call it - and everybody heaves a sigh - the dreaded forests are behind them. Another three days through clear, easy country, and your destination will have arrived.
I suspect now will be a good place for the GM to politely ask the PCs where they would expect to be around midnight that night.
Encounter 1: Ambush at night
At midnight that night, the Fann Yar is at her most relaxed. Double watches were imposed during the trip through the Forest; tonight, only a skeleton crew man the ship while most of the crew catch up on lost sleep. The captain is particularly pleased: "No more 'f these mangy river-rations, lad; tomorrow we break our fast in the beautiful village of Farstretch; ol' Mrs. McCoo makes the loveliest sunflower cakes, an' that's all I'll be eating for the next month, I can promise you!" Dreaming of Mrs. McCoo's cakes, you fall asleep/sit at the stern staring at the stars/talk to another PC of this and that.
At around midnight, the encounter begins. The PCs might be in their cabins, in which case they might not hear anything at all until the attackers have moved into the lower decks. If they are on the main deck, they will have a chance to hear the softly splashing of boats in the river, and to see the ropes being thrown onto the ship. There is no warning, no preliminary attack with arrows. The first few halflings to leap on board the ship try to move about in the shadows without being noticed so that two boatfuls might climb up together; once spotted, they rush the skeleton crew and PCs with fearsome cries. Battle ensues.
The GM should carefully control the flow of battle so as not to confuse new players. Confused players should start underdecks, where one or two halflings might be sent to steal anything worth stealing, unprepared for proper combat. Players on the main deck can be helped by having the halflings take a while to climb onto the boat, leaving four or five low-level halflings to make the first assault. The halflings will focus on attacking the crew members, who are - after all - a whole lot scarier than the PCs at this point. An interesting variant would be for most if not all of the crew to already be dead by the time the PCs wake up, leaving them stranded underdecks and forced to fight their way out.
It is perfectly okay to lose this encounter; one of its purposes is to throw the PCs into proper combat against deadly foes right at the start of the combat. After a while, an imp will appear out of nowhere and communicate with the leader of the halflings; after a short argument, he will leave. If the battle is still in progress, he will return later with a powerful flying creature.
I'll continue this when I next have a little free time :-).