While the upper level of The Sunless Citadel presents a typical dungeon-delving experience, the lower level shifts gears into one of horror. The concept of evil plant-creatures is already creepy, and the body horror of the transformation of Sir Braford and Sharwyn is very unsettling. Belak fits the trope of “mad scientist” quite well, and this section has the adventurers discovering his experiments in the first set of chambers, before confronting him before the Gulthias tree.
Exactly how much you want to emphasise this theme is up to you. If I were to play up the horror elements, I’d describe the chambers as being shadowed, with the monsters emerging from the gloom. Instead of just saying “You get attacked by two twig blights”, I’d say “Two bundles of thorny twigs, twisted into the mockery of children, rush out of the shadows towards you!” Your choice of words makes a significant difference to how the players feel about the threats.
That said, if the players aren’t receptive to the horror of the situation, don’t force it. Ultimately, your players will let you know what type of adventure they’re the happiest playing. This transition can be somewhat jarring, as this horror element is not present on the upper level.
There are several intelligent creatures – mostly goblinoids – in the grove level. Some of them, such as Balsag (area 43) are written primarily as combat pieces and have little interest in negotiation. They want to kill the intruders. The other goblins may negotiate. To play up the horror element, you can role-play them as slightly unhinged; their experiences with Belak’s experiments having sent them to the edge of madness. Perhaps the goblins here are very loyal to Belak, seeing him as the best hope the goblins have of Ruling the World – or a similar, grandiose plan. Or perhaps the goblins serve only out of fear, terrified of what Belak might do them if they disobey. Little verbal or physical tics can help portray how these goblins are different from those above.
Belak himself is not designed to engage the adventurers for long. Assuming he isn’t attacked immediately by the players, he’s there to explain the plot, let out a few maniacal laughs, and do the big reveal of the fates of Sir Braford and Sharwyn. If your players decide his plans sound fine and they want to help him in his work, you might need new players. What Belak gives the adventure is an ending point; he’s the obvious Big Boss, and the players feel good about ending his threat. The battle against Belak and his minions is very dangerous and needs to be judged carefully to avoid a TPK. While I am not averse to Total Party Kills when it is the players’ fault, I’m a lot more careful in big story-driven fights. In particular, if the players realise they’re losing and run for it, I’ll allow them to flee. This provides you with the question of what changes to make to the dungeon for their next expedition.
The answer comes from Belak’s previous actions: Belak wouldn’t consider the party a threat and would go on working as he had, although a few slain twig blights and goblins would be replaced. If Sir Braford or Sharwyn were slain, they’d not be present in the next encounter, but an unconscious or dying PC who was left behind may likely become a servant of the Gulthias tree.
The adventure gives no way to save those who become thralls of the Gulthias tree. You can invent a quest for something to reverse the effects, but I’m happy leaving it as incurable and thus playing up the dark aspects of the D&D world.
The various hints of the previous purpose of the fortress – that is, all the references to Ashardalon – aren’t paid off in the Tales from the Yawning Portal version of the adventure. In the original 3E adventures, further hints at Ashardalon would reoccur in several later adventures until the level 18-20 adventure, Bastion of Broken Souls revealed the true fate of the dragon. Likewise, you can find more on the origin of the Gulthias tree in Heart of Nightfang Spire. You can find both of those adventures on the DM’s Guild if you are curious. The Gulthias tree and the twig blights made a reappearance in the recent 5E adventure Curse of Strahd.
With the defeat of Belak, the tale of the Sunless Citadel comes to an end. What next for your adventurers? You can use the scroll referring to Khundrukar (found in area 37) to guide the adventurers towards the next adventure in the book, The Forge of Fury¸ which was the next adventure in the original sequence. Perhaps an opening in the rift leads into the Underdark and the start of Out of the Abyss or an adventure of your own devising? The Sunless Citadel may have ended, but the adventures of your characters have just begun!