Tome of the Old Ones
Wondrous item, artifact (requires attunement by a warlock of the Great Old Ones).
Gifted to you by your patron, this tome is filled with knowledge of the Far Realms, the Old Ones and strange magics, compiled by the followers of the Great Old Ones. Such knowledge is so incomprehensible that any who do not follow the Old Ones that attempt to read its contents will be driven permanently insane.
To touch this book is to drive yourself mad. Any creature who willingly touches this book who isn't attuned to it takes 4d8 psychic damage every second they are touching it. If they drop below 10 hitpoints, they must make a wisdom save (DC25). On a failure, the creature becomes permanently insane. It becomes a catatonic wreck, unable to move or understand others and speaking only in gibberish. It can no longer take actions or reactions and automatically fails any ability or skill check. This insanity can only be removed by a Wish spell. Any creature who dies from this damage has its soul devoured by the patron who granted this book. This does not affect aberrations or those who worship the Great Old Ones.
To attune to this tome, you must spend one week locked in a dark room performing a ritual. On completion of this ritual, you suffer 5 levels of exhaustion for 1d4-1 days.
When attuned to it, the book may be summoned using a bonus action. It may also be dismissed into its demiplane in the Far Realms as a bonus action. It remains with you only as long as you sacrifice one creature to your patron every week. If you fail, the book disappears and cannot be summoned again until you regain favor with your patron.
If you die while attuned to it and are not resurrected within two weeks, your soul is consumed by your patron, and you cannot be resurrected.
As long as the Far Realms exist, the book cannot be destroyed, it simply returns to its demiplane. It has 200 hitpoints, and if these are reduced to zero by radiant damage, the book turns to ash and reforms in 1d10x10 years (any attacks must target the book itself and not the creature holding it). If the Far Realms are destroyed, the book turns to ash permanently.
Adjusted Ability Scores: One ability score of your choice increases by 4, to a maximum of 24. Another ability score of your choice decreases by 2, to a minimum of 3. The book can't adjust your ability scores again.
Mark of the Aberration: You acquire a physical disfigurement on attunement. For example, your skin might become slimy or your eyes might become a different color.
Control Aberration: While you are attuned to the book and holding it, you can use an action to cast the Dominate Monster spell without using spell slots on an aberration (save DC 18). You can't use this property again until the next dawn.
Tentacles: You may cast Evard's black tentacles at 4th level without using spell slots a number of times equal to half your Charisma modifier rounded down. When you cast it in this way, you take 1d12 psychic damage. You regain all expended uses on a long rest.
Dark Lore: You can reference the Book of the Old Ones whenever you make an Intelligence check to recall information about some aspect of the Far Realms, such as lore about the Great Old Ones. When you do so, double your proficiency bonus on that check.
Wilt: Creatures with a challenge rating of 0, as well as plants that aren't creatures, drop to 0 hit points when within 10 feet of the artifact.
Minor Benefits: While attuned, you:
are immune to disease
gain a +1 to armor class
have resistance to necrotic damage
Minor Detriments: While attuned:
all holy water within 10 feet of you is destroyed.
you deal 1d6 necrotic damage to any plant you touch that isn't a creature.
you cannot deal non-lethal damage.
Favor of the Old Ones: While attuned to the book, you gain 2 extra spell slots of your max level minus two that recharge like your other slots. You can also choose to regain one use of one of your expended Mystic Arcanum. Once you use this feature, you may use it again on a long rest.