You could start the quest by saying there's a disturbingly high amounts of magic emanating from the manor, and when approached from the sky, something tries to keep you away. The hedges are 30ft high with bramble in between them. They also take a long time to burn through due to how thick they are and the fact they are enchanted with abjuration spells that are dispelled once everything in the upper maze is killed.
The entrance guardian is a robed figure, with impossible to see facial features, who warns them about going in, and if they try to fly over with him there, he will cause a gravity-like ability where he forces them onto the ground before they reach the maze. He will say something is hidden under the manor and is sealed, and the party cannot turn back if they plan to go in and go too far. If the party still wishes to go in, he will test them in two ways: Mind and Body. He will ask two riddles, followed by a third one that involves him asking if the party will kill something knowing that what they were doing was their job. If the party answers yes or no, he will accept the answer. He will test their body by fighting the party, but don't give him an HP amount. Make him cause nothing but attacks that deal non-lethal damage (don't tell them its non-lethal) and knock out the party members one by one until half of the party is knocked out or until they retreat. If they retreat, he will heal the fallen and tell them to leave if they do not strengthen their resolve. If he defeats most of the party, he will let them in as he believes that they can stop whatever is inside due to their resolve alone.
I would say have the maze on the upper part be filled with fey creatures that can pass through the walls of the maze without problem, and that they are able to be reasoned with as long as the party doesn't kill any of them, but they will attack, as they are guardians of the lower caves. They are trying to keep something in, and because its been a long while, the fey creatures actually forgot whats inside; they just know its bad. If reasoned with, or if they're genocided, they will either give/have on them a page with a riddle. This riddle will solve a puzzle that will discern certain patterns in the lower maze that have secrets, shortcuts, or alternate pathways (some riddle that pertains to a certain carving that can be interacted with by doing what is listed in the riddle, like rubbing blood on it, touching it with silver, etc.). The fey will also have ranged attacks/spells/abilities that can attack those who fly. There are also a few places in the maze for those inside the maze who will spot indents in the floors which hold loot from defeated enemies. The fey don't care if it is taken, but will bargain for the location of its whereabouts. These spots can not be found flying as they are small details and the hedges are 30ft high as said before.
The manor has a basement that is locked with two arcane locks that require keys to open them. The door is solid adamantine with magical wards to make it nigh indestructible, with an inscription that reads that a powerful enemy, known only as the Fallen One in the writings, is locked under there and that opening the gate is the bad idea. Those with detect magic can see the magic that was radiating the place is coming out of the basement, and seems to be oozing out of the keyholes and cracks on the door.
Navigating the house plays around a lot of traps that you wouldn't find in a regular house, with the house's goal is ultimately prevent you from getting the keys. The first key is in the master bedroom and the second key is in the kitchen. I don't know if you'll have to edit this, but the house is riddled with ghosts, specters and other ethereal/incorporeal undead. The one guarding the kitchen key is a poltergeist that throws knives, cleavers, pots and pans at the players, and the one guarding the key in the bedroom sits at a poker/card table and demands that in order to get the key, you must play his game (your choice). The players can fight him, but expect that he will be harder. The Gambler (as I will call him) will gamble the players for their souls in a game of your choice, but not at once; he will offer them the key in six pieces, and their souls will be split into two pieces per player (or more or less for both, depending on how merciful or cruel you feel). The players do not lose their souls, even if they lose their chips until the last chip is taken. They can opt out at any time and return, but the chips will remain on whatever they have. If he holds the full part of one persons soul when they opt out, they lose their soul, but it can be won back. The party loses if everyone loses their soul. There will be ample artwork loot here, and the ghosts and such that wander the mansion were people who were trying to get into the lower labyrinth, save for the poltergeist who was a spirit conjured to protect the key and the gambler who is the former owner of the manor before the labyrinths were built (the gambler will explain this is talked to while playing his game who is happy to explain. He will not say anything about the labyrinths or what dwells in them, as he truly does not know. he will not even acknowledge the door down either).
Once the gambler is "defeated" (by either means), the party can take a long rest in his room as it seems safe. The Gambler will offer them rest when they finish his game and come to them if they are low on health in an encounter anywhere inside his house after his game is done as a reminder.
Once both keys are taken, the guardian at the entrance to the maze will appear door and stop the party from advancing. If the party answered that they were not ok with killing someone because they were doing their job and they killed any of the fey or the gambler, he will question the party on that, and will give the party a magical yellow stone. If the party said they were ok with killing something for doing its job, and they killed the fey or the gambler, he will commend them for their honesty but say they are walking down a bad road still, and give them a red stone. If the party said they would be ok with it, but didn't kill any of the fey or the gambler, he asked why they lied to him and give them a blue stone. If the party didn't agree and they killed none of the fey or the gambler, he will commend them for their honesty and their mercy and give them a green stone. He will explain that the stone will help guide them. The stone is magical and gives off light when used. He tells the party they will need that to descend to the deepest parts.
When the party goes into the lower labyrinth, they have several floors to descend. Each floor has a guardian that is not at the stairs, and each one has a stone of differing colors. One is specifically hidden behind one of those secret ways that the riddle talks about, which is the green stone holder who is a dragon of sufficient level but still the most challenging of the four and is on the last floor. The blue stone is held by a devil or demon of sufficient level that is on the second last floor of the lower labyrinth. The yellow stone is on the second floor and is held by a vampire of sufficient level who is easier than the demon/devil. The red stone is on the first floor of the lower labyrinth and held by an elvish spellcaster that is to be the weakest of the three. If the party has the stone of that color, they do not need to fight that boss, but they can still find it and defeat it. The party's actions made the maze easier or harder to deal with.
Once all four stone are recovered, the final floor has an altar that has an inscription telling of this dark one, and the magic radiates from it heavily. Players who use detect magic can actually use this to skip all four floors of the inner labyrinth and go straight for the stairs, as the magic radiates like a miasma from the altar and tries to escape outside. Putting the four stones down lowers another set of stairs, with the walls of the stairs telling of some horrible deeds done by a very powerful being. The guardian will appear one last time at the altar room and he will offer to heal the party as if they had a long rest if they obtained the green stone (otherwise, they will have to go back to the gambler's room). Regardless of the green stone, he will tell the party that this is the last chance to turn back, and if they do, the party can return, but they will have to fight the guardian who has all four stones on him and is a sort of punishment boss for not following through, being almsot as hard as the final boss.
The party makes it down the stairs to the lair of the Dark One, and they find inside a weakend but still powerful (and obviously altered) fallen angel, where he thanks the party for waking him up and then he fights them. I'll leave it up to you to fill in what he does, but make sure he is unable to fly because he is too weak and just woke up.
Assuming the party succeeds, the guardian congratulates the party and disappears as he explains he was there only to allow in someone who was able to defeat the Dark One. He disappears after he opens up a huge loot room inside the Dark One's room which holds several powerful items that were used to lure the fallen angel in here, as well as a lot of gold, gems, and other things. Most importantly, it has an entire blue print of the labyrinth, and tells the party of any secret loot spots they have missed.
Sorry if this might be too long, but this was fun to come up with