Great work man; I am totally stealing this!
What order are the encounters to be...er...encountered?
What is the terrain for each encounter? Rooms in a tower?
Where do the short rests go?
How would you adjust this adventure for five 11th level PCs?
The PCs are all human except the Bladesinger:-
* eladrin bladesinger wizard 11
* paladin 2/tempest cleric 9
* assassin rogue 3/shadow monk 6/GOO warlock 2
* berserker barbarian 3/fiendish bladelock 8
* swashbuckler rogue 7/dragon sorcerer 1/battlemaster fighter 3
In my mind I had it as a tower (shaped like a rearing black dragon carved into a mountain) with a walled semi circular compound around the base. When the PCs arrive they are teleported several 100 feet from the outer walls. Its nearing dawn and the whole area is clouded with an acrid fog (the terrain is quite swampy) limiting visibility (imposing light obscurement, and limiting LOS to around 60').
The inner and outside walls are all warded with magic and lined with lead, stopping any divination magic from penetrating them (to protect the delicate negotiations and discussions from prying eyes).
The PCs have 3 hours (1 hour of adventuring time, and time for 2 short rests). If they dawdle outside the tower or take too much time inside it, they might be left time for just the one short rest.
Encounter 1 was the orcs. They guard the outer compound and can retreat inside its fortifications if needed. They attack in three groups (2 lots of 1 Orog, 1 Eye and 5 Orcs, and 1 lot of the Boss riding the Wyvern) if the compound is breached. The Eyes cast Bless (on themselves and the Orog and the Boss and Wyvern) using a 2nd level slot, and then tend to hang back with spiritual weapon. The boss uses his warcry and they all close to melee as soon as possible if the walls are breached, with the boss + wyvern concentrating on any obvious casters.
Encounter 2 is the giant + wolves inside the mountain. I imagined a great hall containing a hostile but disintrested Giant and his wolves. While primarily a combat encounter he is also an optional social encounter (he only guards the tower due to being abducted by the black dragon, and his family killed by the drake). He doesnt really care about his job, but only does so out of fear of the dragon. He's initially unfriendly but might be persuaded to turn a blind eye to the adventurers, and give them advice on how to find the dragon if the PCs break off combat and try and talk to him (substantial bribes help, as does a promise to slay the dragon).
If combat breaks out, the wolves are spread out to avoid AOE's and concentrate attacks on one or two PCs looking to knock them prone so the giant can attack with advantage. The wolves are well trained and are not above using the Help action to assist the Giant as well. The winter wolf concentrates his breath weapon on any high AC foes (your bladesinger) and isnt afraid to catch the giant in the radius (he's immune to cold).
The PCs can rest now having cleared out the outside and the inner compound. The Orcs are too savage to alert anyone, and the Giant doesnt think to do so.
Encounter 3 is a trap that guards the upper level. Set DCs as desired, but as the PCs walk up a long sloping coridoor to the next level, they set off a trap revealing false walls either side of the party which slide up revealing (inside each wall flanking the party) a Helmed horror - but in place of a helm each horror has a grinning flaming skull! On round 1 (likely while the PCs are surpsised) the heads detach from the armor, and both fireball the party. The horrors engage a single target in melee (their swords deal +1d6 fire damage, AND they are immune to fireball as one of the spells they are immune to). The skulls then hammer targets with magic missile and fire rays.
Once this encounter is dealt with the party find themselves on level two. The next encounter (number 4) is the Hook horrors who the illithids have placed in stasis via meld into earth (they brought scrolls with them). It takes the beasts a round to emerge from the walls (unless you feel like surprising the PCs again!) and they begin combat surrounding the PCs. This should be a rather stand up fight.
After clearing this level the PCs have the option of short resting again (rememeber they only get two!).
The next encounter is the Illithids and company. I suggest changing up its spells (ensure it has counter spell, an AoE and shield - and wall of force!). It's not above placing a lone thrall as a guard away from its lair for the party to encounter solo - if the thrall dies the Illitihid becomes aware of the PCs (when its mental bond breaks) and it enters this battle with a buff spell or two up.
It makes use of wall of force to split the party so it can deal with smaller groups at once. AoEs, mind blast etc and thugs + the umber hulk make this a dangerous fight.
Back in a second with the Mezzoloth and dragon fights.