Howdy! Let us see what we can put together for the Sunless Citadel… Spoilers ahoy for anyone who doesn’t want to know! So read no further if you want to remain ignorant!
So, you start in a town, so Brindle Town is your go-to place. Maybe add in Friendly Tavern if the adventurers end up in one (because when do they not?). Then they go to a buried fortress. Dungeon Depths works very well for that (maybe add some Spooky or Elements - Horror to punch up the freakiness), and then there you can use the Kobold Lair (which also has some traps) and Goblin Fight for battling those critters. Maybe a little Witchwood to give it a more “outdoorsy” feel.
Then you have the twig blights themselves. Now, as Syrinscape has yet to have many plant villains, I had to build it up, so you’ll probably want to put it in a Custom Mood. I used the Skravling Attacks from Skravling Battle (for their bloodthirsty bites), baby dragon scratches from Dragon Battle, treefalls from Storm, and (bear with me here) the fireplace element from Friendly Tavern. The snap and crackle of the fire, when not turned up too high so as to avoid hearing the subtle roar of the flames, sounds a lot like crackling twigs! Also The Spires of Xin-Shalast - Whispers on the Wind have the elements Horror Tree Steps, which would be great while they’re moving.
If the group ends up facing the Dragonpriest Troll, use the Ogre or Ettin soundsets, and there are already soundsets for Giant Rats and Undead, should those end up wandering into the party’s path. For traps in the citadel, Dungeon Depths has some trap sounds, as does Burnt Offerings’ Thistletop and Kobold Lair. There is Bugbear Battle, of course, for any bugbears, though you may have to turn off the larger-scale battle sounds for a single enemy encounter. For the thoqqua - Fire Elemental set. For the shadows in the shrine, try Spooky or Haunted Desert House. The water mephit might best be served with Burnt Offerings Catacombs of Wrath, specifically the Battle in the Cathedral (minus the sinspawn sounds, just keep the Erylium sounds - she’s a quasit and has a lot of the same size and features as a mephit). Flying Fiend might work well too. Grimple Battle has an acid breath one-shot for the water mephit!
For Sharwyn, you could use the Sorceress - Spellcaster set from the Boardgame player, or the Ezran - Male Wizard set and just skip any of the magical incantations. For Sir Braford, use Valleros, Male Fighter. For the white dragon wyrmling, Dragon Battle, specifically the baby dragon sounds. For the breath weapon, there is a White Dragon set with a breath weapon, but it sounds a bit “big” for a wyrmling. You might try the Ice Spell universal one-shot as an alternative. For Kulket, Belak’s animal companion, the Swamp soundset had some frog sounds and frog one-shots.
For Belak himself: there is a Druid Battle in Strange Aeon’s The Thrushmoor Terror - Thrushmoor. It has some really freaky and unsettling background music and moods, which fits the Sunless Citadel really well. However, the druid in question in that set is a woman, and a lot of the sounds are fire crackling and wolves howling. What might work better is the Elements - Horror set (for eerieness), and some of Erzan - Male Wizard’s sounds. He has cloth armor sounds (since Belak is only wearing a cassock) and can give you male yells and incantations. Then use the cleric spells from Kyra - Female Cleric for the druid spells (most of them are on there). If it comes to hand-to-hand combat with him, Kyra’s scimitar sounds work well for Belak’s sickle.
Here is a link to a campaign I knocked together. All you’d need to do is the Custom mood for the twig blights themselves, and you’re ready to roll! https://syrinscape.com/account/campaigns/6678-b019b4e1f6/