This is not the first time the party has had to retreat. Thinking about it, the previous two sessions before this one had them retreating from bad situations. (There have also been times in those two sessions where they've totally destroyed the opposition, so there is variety in the encounters).
This campaign is, without a doubt, the most hostile I've yet run for the PCs. The BBEGs they are fighting are way more powerful than them, and as a result the poor County of Ulek is being overrun. (Imagine a nice warm mediterranean country suddenly being in the middle of a blizzard - eek! I really should hunt up some Barbara Hambly books for additional inspiration).
The setting is very light on arcane and clerical magic - more bards and druids than wizards and clerics. A lot of regular D&D options don't exist. (The party has a druid/wizard, bard/druid/rogue and incarnate as its magic-users).
On those rare occasions I do get to play D&D, as opposed to DMing it, I have a fairly paranoid, old-school play style. During a recent Greyhawk Ruins campaign, the other players quickly learnt that when I started retreating, it was a good idea for them all to retreat. Often, I'd retreat at the first sign of an encounter, just because I didn't like the odds. I will also say that I was a lot braver once I reached higher levels, say 9th or so, and had magic to get me out of situations that turned bad! The king of the situations which went bad was when I was facing a Great Wyrm Red Dragon alone! Oops. I didn't have any magic in that situation, but actually managed to talk my way out of it. By the end of that campaign, I was the only player who had never lost a PC.
This encounter was lifted fairly much straight from the Corum books, and was designed to introduce a new class of enemies - the Brothers of the Pines. In my original notes for encounter, I thought I'd send them up against a group of such enemies, but reverted in the session to just having the one. (Later sessions will have the groups of lesser enemies).
One feature of the encounter is that it shows that you can't just ignore things. There were four things I included in it that should have alerted the party that something was unusual:
* In the round before Hew Therch was slain, the Druid's bear tried to grapple him... only to fail because he couldn't be pulled off the horse.
* When Hew was slain, he fell off the "horse", and it began to drag him with its teeth away.
* The horse was tough. As in, AC 31, DR 10/slashing, immune crits, and dealt 1d6+10 damage with each hoof attack. And hit most of the time.
* The "horse" would provoke AoOs to attack the healers and lightly armoured members of the party.
(What CR was the horse? I'd put it about 12 or 13).
One comment I got from the players was that the PC that went down was killed by a critical, and that couldn't be predicted. I disagree. Any creature that attacks 3 times/round is going to have its fair share of criticals, especially over a combat that takes 12+ rounds. For superior play of D&D, you've got to keep the possibility in mind.
I strongly believe in heroism in D&D, but I also believe that you need to pay attention to what's going on, and having tactical plans for situations such as this. It took three rounds from the first PC suggesting the retreat take place before that actually happened.
Cheers!