Well, it happened - TPK, due to .... everything.
I've been trying not to fudge rolls - maybe I should have done. When the party strayed into an area patrolled by undead (a kind called 'Brothers of the Pine', which are a sort of druidic forest zombie about as tough as Ghouls; I found them in old White Dwarf scenarios but I don't know if they were ever officially adopted, eg in 'Fiend Folio') I rolled a random encounter and... a lot of them turned up. Too many for the party to deal with anyway. Maybe I should have tried to adjust the numbers or give them another way of avoiding the encounter or both.
The party didn't capitalise on the advantages they had - the Brothers of the Pine are particularly susceptible to fire and the party had a campfire; if they'd taken burning branches they would have had fairly efficient weapons against the Brothers. Only one of the party attempted to do something like that - throwing his flask of oil at one and attempting to light it. Great idea on Tuh's part, but he failed to do so first time. Throwing the burning brand probably wasn't as good an idea as holding one end and fending off the flammable undead with the other.
Jericho the Elf used his Magic Missile on one; he could have probably used his Web scroll on some of the others, but he didn't. That meant that within a couple of rounds the whole party was under sustained attack, instead of trapping some and dealing with the attackers piecemeal.
The Brothers kept rolling high for their attacks - several 18s and 19s - while the party kept rollings 7s. Not that it mattered, the party didn't have enough magic weapons to deal with the Brothers anyway. The hits they did manage - Grrk's strike with his shortbow against one for example - didn't work. When Sir Norrin hit one with his magic sword, which should have worked fairly well (not as well as fire though) he didn't do very much damage because he rolled low. Fire was their best friend in this encounter and they didn't use it.
One after another, the members of the party were cut down under the swords of the Brothers. First to fall was Bork, then Tuh, and finally (as far as the PCs were concerned) Jericho. After a moment of stunned silence, and then some laughter, the players wanted to see how Sir Norrin, Grrk the Squire, The Mystical One (First Level Cleric and therefore no use at turning them, in the description I've got it says they're equivalent to Spectres but I pegged them at Ghoul status which is still really tough at First Level), and the permanently-injured Mohag the Wanderer managed to fare.
Not well was the answer. PCs and NPCs all butchered in a couple of rounds and, according to the description, ready to be filled with sap and turned into new Brothers - and Sisters in The Mystical One's case - of the Pine. A combination of my bad DMing - sticking to the Number Appearing roll when I knew that it was going to be very tough - the party's panic - not using their spells as well as they could and not capitalising on the fire as a source of effective weapons - and just unlucky rolls meant that everyone died.
So, the surviving members of the party were Yor, the Magic User who fled in session two, and Betsy the horse, who ran away in the midst of the fight with the Brothers. She will, presumably, after many adventures, turn up at South Reach (Sir Norrin's manor) riderless, and with a sack of Giant Spider-legs in her saddle-bags, much to the consternation of the servants there.
The lads have rolled up new characters. They've also decided that they want a trip to the sea-side so these PCs look likely to be leaving Threshold and the mountains behind them and heading south towards the coast. It's a wild place, the Grand Duchy; they won't be short of adventure as they travel through it. Hopefully though they might get to the seaside before they all die.