Sylvanwroth's Library

The library is vast, seeming to stretch impossibly further than the dimensions of the hall it occupies. It's a labyrinthine space of halls and alcoves filled with towering bookshelves that rise like ancient trees in an enchanted forest, shrouded in a semi-permanent twilight. Islands stacked with books on all sides rise between walls of thick tomes, scrolls under panes of glass, and other items harder to describe - nested dolls, strange crystals with protruding wires, puzzle boxes made of brass and copper. 
The ceiling at first glance is open to the night air, but you're hundreds of feet below ground and most of these books could hardly survive exposure to any amount of wind or rain. Above you instead is an intricate illusionary masterpiece, an exact replica of the night sky with stars glimmering faintly, misty clouds, the lavender moon, and as you watch, a shooting star. These seem to accurately reflecting celestial events in real-time excepting that the moon seems larger and closer that it ever does on the surface, casting pinkish light over everything in the great hall.
The air is heavy with the scent of parchment, ink, and the faintest hint of some kind of astringent cleaner – but markedly neither dust nor mold. This is a testament to the countless years worth of knowledge contained within these walls and the infinite patience and attention of skeletal servants to clean and maintain it. The temperature is strangely neutral, seemingly maintained by magic to best preserve the countless scrolls, books, and parchment in the library.
Books of all shapes and sizes line the shelves, some appearing ancient and others surprisingly modern. Several of these are under glass, presumably kept in total vacuum, and the fact that it feels hard to catch your breath suggests the oxygen levels are kept artificially low in the whole of the library as well, as an added defense against decay. A number of tomes float in mid-air, opened by spectral hands that seem to flip pages. In other places, small orbs of light float gently in place, providing illumination for the reader. These orbs cast long, dancing shadows across the room, contributing to the eerie but fascinating ambiance of the library.
There are areas set up for different interests: one corner hosts a massive map of the cosmos, surrounded by astronomical equipment. Another area seems alchemical at first, but brief investigation reveals it to be a workspace for the care and preservation of the books themselves.
Nestled between the shelves are a series of comfortable reading spaces, equipped with plush, antique chairs and sturdy tables. Around the hall one can also see various small, spotlessly clean brick fireplaces, presumably magic as they burn with an azure flame. They give out a cozy dry heat and cast a calming light in each area.
At the far end of the library, you see a massive blackstone desk worked with bizarre symbols that resemble Celestial, if anyone is familiar with the language, but an archaic form of it unknown before now.  The surface is covered in parchment and quills, illuminated by a larger orb of pure red light. Behind it sits a throne-like chair studded with gems that you recognize as the - or at least, a - throne of Chaos, and it is here that you sense the ancient drow lich might spend a majority of his time.
Despite its scale and the clear power of the being who presides over it, the library is obsessively clean and strangely welcoming, with as much care and passion given to these books as to the plants in the Conservatory. This is a sanctuary, a place of knowledge and study yes, but a safe place for the knowledge to rest. Yet you also feel a note of caution. There's an undercurrent of power, a reminder that the being who controls this place has had the time and ability to master secrets beyond mortal comprehension.

Design

The Library occupies about 80x80 onsite and another 7,000 square feet in the cloud, with most of the internal surface area taken up by shelves and the books they contain. The ceiling uses a mirror illusion which accurately reflects - in reverse - the night sky directly above the central arch of the actual tomb, and effectively hides the border between the local and extra-local dimensions of the space.

Entries

The physical library has 5 egress points, and can also be entered and exited through the extraplanar space into the Border Ethereal with spells or items with that ability.

Denizens

• Morvain, the keeper. Morvain is a creature known as a "Grimorin," a mystical being born from the intermingling of dark magic and ancient knowledge. As the chosen custodian of the lich's library, Morvain appears as a swirling locus of  wispy tendrils of dark energy emanating from a humanoid-shaped, levitating form. it has long, elegant limbs and its eyes glow with an otherworldly luminescence. Morvain's skin when visible is pale and translucent, revealing the faint traces of runic inscriptions that flow beneath the surface. It is bound to the library, ensuring the preservation and protection of its arcane tomes, while also serving as a guide for those who seek approved or forbidden knowledge within its hallowed halls.
• 1d4+3 leather scaretakers are in the area at all times, armed with shortswords but all on work duty caring for the tomes. They understand common and will comply with reasonable commands that don't interfere with or contradict their orders, including 'bring me X book'. They are free willed and can be malicious towards those who mistreat them, able to cause a paralytic terror at will that will root the victim to the spot on a failed charisma save DC18. If that warning alone is not enough to stop any misbehaving, they can actively drain life energy conforming to the Life Drain ability of a Wight.

Contents & Furnishings

The contents of the Library consist of mundane and magical works, and the magical works can be further characterized as spellbooks, wondrous books, or enchanted books.

• Mundane Works: An eclectic selection of well over twelve thousand titles, of which perhaps three thousand are works of fiction and the rest, nonfiction on a variety of subjects, including reference works, historical accounts. travel journals, overviews of flora and fauna of various types, language books and dictionaries, books teaching various subjects such as jewelry making, weaving, tanning of leather, etc, and biographies of famous people; s few of these are mundane books but concerned with magical subjects, such as Yuan ti rituals, identification of magical symbols, and travel guides to various planes and other exotic magical locations.
• Spellbooks: Sylvanwroth has many hundreds of these. Best not to think too hard about how he came to have so many. Tucked in among the 'modern' spellbooks are those containing elder spells that may or may not still function. Safe to assume that Sylvanwroth has access to every spell in his library, new or old.
• Wondrous books: These include the magic item books in the PHB and dozens more of that nature. He has multiple copies of most of these, stored under various conditions as these books have a tendency to wander the planes if not in active use; some have been in active use and are recharging over the next few hundred years (Sylvanwroth can wait).
• Enchanted books: These are like the mundane books that deal with magical topics, but they are also themselves, magical in nature; these include items like the Predictionary, the Clone Cypher, and other books that deliver specific magical effects.

Because there are so many books in the library, filed using a system only Morvain and Sylvanwroth fundamentally understand, it is no easy process to locate anything specific, and a book at drawn at random has a 95% chance of being something mundane simply because there are so many more of those compared to those of a magical nature. Every point of wisdom bonus increases the chance of finding something specific or a magical tome by an additional 5%.
If a magic book is located at random, Roll D6 to determine the type:

• 1-4: Spellbook
• 5: Wondrous Book
• 6: Enchanted Book

Valuables

Contents of Note:

• Tome of Clear Thought: https://www.dndbeyond.com/magic-items/4781-tome-of-clear-thoughtSylvanwroth has used it three times so far (+6 to intelligence score), but it retains its magic as it has been over a century since the last time he used it.
• 3 x Tome of Leadership and Influence: https://www.dndbeyond.com/magic-items/4782-tome-of-leadership-and-influenceSylvanwroth has maximized his charisma score (30 Charisma score), having used the book 7 times, and made two copies since. One of the books has 2d8 years remaining, the other two are primed for use.
• Tome of Understanding: https://www.dndbeyond.com/magic-items/4783-tome-of-understandingSylvanwroth has used this tome four times, raising his wisdom score to 22. The current copy has 2d4 years remaining.
• Arcane Grimoires, various: https://www.dndbeyond.com/magic-items/2411893-arcane-grimoireAll three varieties are in the collection, with several examples of each, many of them themed towards a certain kind of spellcaster. Sylvanwroth uses an Arcane Grimoire+3 personally.

Hazards & Traps

Any sort of shenanigans with the books will be met with lethal force from Morvain and the scaretakers. Shenanigans include attempting to remove any materials from the library, damaging or defacing books, or any other nonsense of the sort. Misplacing a book into the wrong spot on the shelves will be met with a stern warning and doing it more than once may elicit a lethal response. Spells or other text, however, may be freely copied, and blank parchment and enchanted inks are available at the reading tables for this purpose.
Many of the books are themselves hazardous as well, especially in the children's book section where there are several copies of The Child Eater which literally attacks on page 3, and Alecta's Journey to the Wonder-Land which teleports the reader and everyone within 15' of them into literal Wonderland, a pocket demiplane of the Faewild which they must navigate through to the end and escape. In other parts of the library there are books such as Paralysis of the Soul which launches black chains from the Abyss to bind any reader who does not first perform a purifying ritual before reading, and many enchanted books that trigger all sorts of strange and occasionally dangerous effects like the Predictionary, Codex Umbra, and Clone Cypher.
The library itself is also essentially a seamless tesseract, and cannot be exited through the same door one came in through. This is such a commonplace fact that it's unlikely Sylvanwroth or his various minions would think to mention it, but Morvain and the Scaretakers will notice if the party is in distress and and will try to assist them as long as it doesn't go against their other imperatives, such as protecting the books from shenanigans. Fighting back against a book counts as shenanigans. Of course none of the restrictions apply to Sylvanwroth, but he is the ultimate scaretaker.