Even as a simple DF adventure, it's pretty linear and doesn't require much thought. However, my players like thinking, exploring, and solving puzzles and mysteries a lot more than they like straightforward combat. They'd just had a few sessions of straightforward combat, and I figured they'd lose interest if I gave them another. Instead, I adapted the premise to create an interesting puzzle monster.
My first step was to declare to myself that an eldritch horror from beyond time and space, the kind of thing that creates a 10 mile morass of floating vines to entrap ships on the high seas, can't be killed by unsophisticated direct damage. I didn't really stat it out, but I was thinking DR 30 (not versus silver), 1000 HP, Instant Regeneration, Magic Resistance 20, and so forth. The massive DR, HP, and regeneration meant that fighting the thing straight up was an impossible battle, while the weakness to silver was the first step towards giving them an out.
The next step was to think about ways to hurt this thing. I knew I wanted it to be vulnerable to silver weapons because a lot of PCs in my games, over the years, have bought silver weapons that weren't really useful and I wanted to make up for that. I could also provide hints to the vulnerability with vague allusions to moon metal or were-bane, or confuse the issue by alluding to night metal or heavenly metal (which most players would think to be meteoric iron). I finally decided on a multi-ingredient poison or herbicide.
I started with belladonna. It's a poison, but it was also used as a cosmetic, and "beautiful lady" is an easy translation into a lot of languages, at least at a recognizable if not accurate level. Garlic was another easy choice. It's historically a cure for just about everything, is the kind of thing that might reasonably be found on a sailing ship, and has a bunch of funky slang names that are generally recognizable. Two of my players are from the San Francisco Bay, so I was worried that I would give the game away immediately by calling it "the stinky rose".
I was stumped for the last ingredient for a while. I wanted at least one ingredient that if the players couldn't figure it out entirely, the delvers would at least be able to find some close substitutes. That made me think of white wine vinegar, since the ships might have white wine to use as a substitute. A wikipedia search on the varieties of vinegar confirmed that vinegar was a good choice, because of all the varieties, and I finally chose Jacob's Tears vinegar as a strange one that had alternate names.
Finally, I wanted them to have to apply the poison to a specific place. The original adventure has the Mother of All living at a bottom of a 100' shaft lined with budding and growing vine horrors. It's creepy, but a bit stupid: the adventurers can start by standing at the top and dropping damaging spells and attacks straight down, while the Mother can teleport anywhere in the Sargasso Sea so she can either move to the top of the shaft and fight the adventurers or teleport the heck away and completely frustrate any chance of victory. That's weak adventure design, since it means the adventurers only win if the monster gets the Idiot Ball. I wanted the Mother to be a static foe, but clearly some kind of creepy producer of additional threats - a mother indeed. I liked the imagery of the original shaft of budding vine horrors, and it had a nice resonance with the literal meaning of kindergarten - "child garden" - which inspired another clue. The heroes would need to shoot the Mother in the kindergarten.
As this was supposed to be a puzzle, giving them a single list of "moon-metal, stinking rose, pearl barley vinegar; apply to the budding vines" would be something of a let-down. Instead, I came up with the idea that each of three ships previously trapped in the Sargasso had tried to create the poison with varying degrees of cluefulness and resources. The various logs' allusions and half-references could be compared to each other to resolve the puzzle. I ran the clues by Bruno to see if it was too hard - I figured the combined mental resources of 5 people at a table had to be at least as good as one very bright gamer. She figured it all out and even realized the Mother was a load-bearing boss.
I was happy, and wrote up notes and hand-outs here.
Actual PlayAs it turned out on the tabletop, I forgot that my players were essentially a committee, and a committee is less bright than the individual members. I had to gently steer them away from some of their more insane half-conclusions, and as it turned out, none of them really knew that belladonna was used as a cosmetic. Eventually, though, they managed to figure out the poison and make it.
When they finally arrived at the Mother of All's rooms underneath a wrecked ship, I had a sudden insight: if there's only one thing to shoot, they're going to shoot that one thing. I probably should have thought of that earlier. So I ended up drawing a bunch of stuff on the map: a collection of venus flytrap-like mouths, a "bunch of weird electrical things, some kind of weird organic Tesla coils and Jacob's ladders", a bunch of weird organic pump-like organs, some "strange polyp like growths", and "a patch of open space in which new vine horrors periodically bud and grow." Then I had to clarify that the pumps were not the thing's heart and didn't look like hearts and for all they could tell, were being used to drain the water from the below sea-level chamber they were in.
The players now had several targets to chose from, and being players, they fixated on those dang pumps. I don't know why. More gentle steering and reminders that pumps are not necessarily hearts when dealing with eldritch biology of some kind of creepy Cthulhuoid plant eventually allowed the players to remember the phrase "essence of motherhood." They couldn't read the word "kindergart" on the print-outs, which was my mistake for putting the blur too close to the word. Though Bruno managed to read it on the PDFs so I'm not sure how I was supposed to catch it. Eventually they shot a silver-coated meteoric iron poisoned arrow (the scout had a collection of silver-coated meteoric iron arrows, so they didn't even have to take the chance on silver versus meteoric) into the nursery, the Mother of All retreated, and we had a fun chase sequences as the delvers fled the suddenly sinking wreck and tried to recover loot on the way.
It was a pretty memorable experience, and a lot of fun. I strongly recommend that other GMs either steal this puzzle or come up with something similar the next time they need to put their PCs up against a nigh-unkillable monster.