I like the idea of linking the Twilight Grove and the Gulthias Tree with the Shadowfell.  Maybe the Gulthias Tree grows back over time as the forces from the Shadowfell reach out and attempt to manifest.  That same magic could preserve the ruins in a similar way. 

Wysperra, in the original 3rd edition modules, it wasn't outrightly said, but reading between the lines, the area to the north west of the citadel was destroyed by a massive red Dragon Ashardlon (confirmed in the module itself), it's likely the destruction Ashardlon caused may have caused the citadel to fall into a natural cave system that was likely hidden underneath entirely by accident likely by causing localised earthquakes during his attacks......

As for the tree, it grew out of a vampires body from a wooden stake which still had life in it, Gulthanis was a lackey of Ashardlon, the citadel was a temple to him......

Quote from Boone_Faustus >>

 In 5E they die 24 hours after the tree is destroyed.

  Hmm interesting to know, I don't have the funds to get the books at present, recently lost my job :(

Quote from Wysperra >>

I have a question about the dungeon itself. The whole thing during some cataclysm got buried and the upper level are what were the outbuildings of the castle? How is it that under all of that there is a courtyard with the tree? I know the answer is, "It just is". but that sounds kind of lame to tell the players.

 the Citadel stands mostly intact due to the dragon cult's protective wards. But still the attacks from Ashardalon's rampage destroyed quite a bit nonetheless. The Gulthias tree is not in the citadel. It is in a grove underneath it. The Gulthias tree can be ruined, but never fully destroyed. Its not a far stretch of the imagination that Belak took the seed, created the twisted grove and re-grew the tree in the 12 years that he's been there. There are also ways to get into the Underdark and its said that the purple funghi around the second floor, used as source of light and goblin nourishment, is indeed from experimentation. In the Arboretums this experiment is taken further where they try to cultivate twisted/underdark flora. Belaks study has a back wall that is broken and opens into a rough stoned cavern that is the grove. The grove was never part of the original Citadel.

I doubt Gulthias, when he was still a vampire, and not the tree with his essence...saw the citadel as a temple. Sure he was a firm believer and participant of the Dragon Cult. However he had his own Spire. The way I see it he came to check on the Sunless Citadel after Ashardalon rampaged...trying to preserve some material and take it back to his own spire and followers of the cult. As mentioned in the followed 3e adventure module.

Quote from Giblix >>

Quote from Wysperra >>

I have a question about the dungeon itself. The whole thing during some cataclysm got buried and the upper level are what were the outbuildings of the castle? How is it that under all of that there is a courtyard with the tree? I know the answer is, "It just is". but that sounds kind of lame to tell the players.

 the Citadel stands mostly intact due to the dragon cult's protective wards. But still the attacks from Ashardalon's rampage destroyed quite a bit nonetheless. The Gulthias tree is not in the citadel. It is in a grove underneath it. The Gulthias tree can be ruined, but never fully destroyed. Its not a far stretch of the imagination that Belak took the seed, created the twisted grove and re-grew the tree in the 12 years that he's been there. There are also ways to get into the Underdark and its said that the purple funghi around the second floor, used as source of light and goblin nourishment, is indeed from experimentation. In the Arboretums this experiment is taken further where they try to cultivate twisted/underdark flora. Belaks study has a back wall that is broken and opens into a rough stoned cavern that is the grove. The grove was never part of the original Citadel.

I doubt Gulthias, when he was still a vampire, and not the tree with his essence...saw the citadel as a temple. Sure he was a firm believer and participant of the Dragon Cult. However he had his own Spire. The way I see it he came to check on the Sunless Citadel after Ashardalon rampaged...trying to preserve some material and take it back to his own spire and followers of the cult. As mentioned in the followed 3e adventure module.

 I expect the Grove may have been part of the cave system that the Citadel fell into, and Gulthias tried fleeing through it into the underdark when those opposing him and his followers (or those that survived the collapse) caught up with him before he could get too far......

Hence the tree in it's underground location.

The original module stated it was a location for the Cult that followed and worshipped Ashardalon, so refering to it as a temple isn't wrong, even if it clearly isn't stated as such in the text.

Who knows what Gulthias thought, he doesn't get much info written on him either in Sunless Citadel or Heart of Nightfang Spire, heck from what I remember reading, only Forge of Fury and Bastion of Broken Souls have any connection to each other, these 4 modules form a loose series, but I don't recall the others fitting in, which was a shame.

I'm going to plant a research log/journal for Belak in his study, but if they don't come to the idea themselves about trying to use the fruit to cure the thralls and/or don't find or read the journal then I'm going to have them make a wisdom check to see if any of their characters would know to try to use the fruit. I think I might have it at DC 15.

Does nobody have a problem with sending 1st level characters to fight a dragon?

Its a young wyrm, in captivity. It has some big damage output, but dies easily as well. In all the games I DM'ed or played through it...never was a problem. If you use milestone leveling the group also levels up after this encounter.

@boone sounds like you're trying to force them into using the fruit.

The breath weapon is quite strong, it could easily take out one or two characters. Its bite can also do decent damage. Did you give it lair actions?

Speaking of lair actions, do they count as the dragon's action?

Its a Young Wyrm, in captivity... giving it Lair actions is illogical. It had no time, nor a chance, to even build a lair. The room you find him in he's just gathering crap inside that room. Not much for a lair to begin with. 

And yes his breath weapon does a lot of damage. With a group of 3 players, Erky and Meepo as NPC's... Meebo was at death's door and 2 players were heavily wounded. The other 2 unscathed. Your players can focus fire and then 35hp is not a lot.

Quote from Giblix >>

Its a young wyrm, in captivity. It has some big damage output, but dies easily as well. In all the games I DM'ed or played through it...never was a problem. If you use milestone leveling the group also levels up after this encounter.

@boone sounds like you're trying to force them into using the fruit.

 Not at all. Simply giving them the knowledge that something can be done isn't forcing them into doing it. They could just as easily choose to keep the fruit for themselves or sell it to the highest bidder as the last fruit from the tree (if they choose to kill the tree).

My group negotiated with the dragon. They got her to help them clear the goblins in exchange for the party helping her to take control of the kobold colony. I have a feeling she'll be making a nuisance of herself in the future.

Meanwhile, the party managed (just) to burn the Gulthias Tree and capture Belak without killing Sharwyn or Sir Bradford. I had the party find an apple in the possession of the goblins but they had already used it to cure one that had contracted a disease from mama rat. (He would only have needed a quarter of the apple but he scoffed the lot.) So they were without an apple to heal either Sharwyn or Sir Bradford.

I allowed them to tap one last magic fruit from the heart of the dead tree. They decided to use the apple on Sharwyn. (Again, they could have divided the apple, but the decision was made by the same impetuous member of the party that scoffed the first apple.) Nevertheless, they were happy with the result - particularly the warlock who was after Shatterspike anyway.

I can't find any official errata for The Sunless Citadel. I just bought it last night and am reading through it now. I am wondering if this is an error:

The door that is carved to look like a dragon has a mechanical lock as well as an arcane lock spell cast on it. The Strength (Athletics) check to defeat the arcane lock is DC 30, and the Dexterity check to bypass the magical lock using thieves’ tools is DC 25.

For the thieve's tools check, shouldn't that be "mechanical" and not "magical?"

Looks like a mistake. I found the DC's to high. Not to mention the Athletics check for an arcane/magical lock? As well as Thieves tools for a magical lock... so what is for the mechanical one? I'd just adjust the DC's and pick the checks you deem most suitable for them.

Where is the best place to report possible errors?

For the DC's, aren't they high in order to force them to get the key in the other area of adventure?

Is the door mission critical or just a side bit? If it’s a side bit it is unlikely that IF a party finds the key they will return. At least it seems unlikely if they defeat the main villains. 

It looks to be a side bit.

Here's another question, how many hours on average does this module take? After reading it, I'm guessing at least 2 sessions of 4-hours each but probably more.

As far as the white dragon is concerned... I'm actually going to have it be a black dragon hatchling instead. :3 To tie in with a later part of the Yawning Portal series.

Quote from Amethystine >>

As far as the white dragon is concerned... I'm actually going to have it be a black dragon hatchling instead. :3 To tie in with a later part of the Yawning Portal series.

 And what would that be?