So, I have a question about one particular section of Thistletop as it appears on the map towards the end of Burnt Offerings. There is an area in the second dungeon level of the fortress - specifically, the waterlogged treasure room labelled 'E7' - that is accessible by just swimming inside, with a DC 25 Perception check to find from the surface and a DC 20 Swim check to navigate the 10 ft wide, 20 ft deep underwater passageway. I'm wondering if any of you have ever had PCs discover and try to use this alternate route, and how that changed the experience of the dungeon.
From what I understand, as long as the PCs have reached 3rd level, they should be alright just fighting Nualia at full strength (and even sneak around her trap leading into the observation chamber at 'E4'), even though they might be missing some of the loot from the upper chambers. They'd then have a huge surprise advantage against the goblins and Nualia's mercenaries. How accurate is that assumption?
My interpretation:
AE, p. 63: "It emerges at the base of the island, about 30 feet underwater, an entrance hidden by coral growth and seaweed that can be discovered from outside by a DC 25 Perception check." I don't interpret "outside" to mean "from the surface." At a minimum, I would expect someone to be underwater to see it. Even then a DC 25 perception check should be no easy task for 3rd level characters. If they're taking the time to search the underwater base of the island, I would expect that to draw attention, notably from the bunyip. Having found the entrance, a DC20 swim check should also be no easy feat for 3rd level characters (armor check penalties will be difficult for some characters. Of course they can take their armor off...) The crab should aggressively defend the tunnel and engage the pc's while trying to swim and presumably hold their breath.
Having found the tunnel, managed the swim check and defeated the crab, they then have to fight through the shadows in E6. Note: the incorporeal undead can only be damaged by magic weapons which I assume are in short supply in the party at this point (spells and positive channeling work too of course.) The shadow encounter is easier for parties coming through "the conventional way" since they can choose when to confront the shadows, retreat if needed (the shadows don't leave their room) and they presumably have Ripnugget's magic short sword and Nualia's magic bastard sword to help out.
But yes, if they manage through all that, they could reach Nualia directly. Not sure there would be a "huge" surprise advantage against the goblins and mercenaries since they are not well positioned or well coordinated defensively in the first place.
I let my players in through this entrance and they did things backwards, with hindsight I reviewed the entrance and they probably should not have been able to.
However they considered the frontal approach a deathtrap with the thistle hedges and bridge so they would never have come in that way.
In practice the killed Nualia , ended up recruiting 2 of her henchman and then an alert had gone out and they had to launch a full assault into the teeth of the alerted goblins which was an impressive massed battle