Sentient items - historically swords, at least in Dungeons and Dragons - are often a fun addition to a party's arsenal of oddities and options. Moorcock's Stormbringer is perhaps the classic example, but it isn't always weapons that are sentient. Fiction gives us Bob the Skull from the Dresden Files, as well as an odd Dragon Amulet from the Return to Brookmere Endless Quest book.
A fun book from my youth. As I recall, it features
Tim Truman art - well worth the look.
Monte Cook extensively covered rules for creating sentient items in the Book of Eldritch Might III (compiled here with the first two books), and to be honest, that was what opened my eyes to the possibilities. (As an aside, I find all of the Malhavoc Press product I read to be inspiring, and if 3.x is your jam, I recommend looking into them).
The point being that anything can be sentient, with a bit of effort and creativity.
In addition to their powers and plusses, sentient items bring their own agendas and occasionally dominate their wielder, forcing the wielder to pursue said agenda. On rare occasions, the item's agenda and the wielder's agenda match, so no domination is necessary.
Dancing Scimitar, Anson Maddocks, 1993
Still, the question of where the sentience comes from is there, hanging in the air.
The rules for 5e have a few suggestions: possessed, haunted by a previous wielder, or self-aware because magic. These rules work, but there are so many more, entertaining options.
I'd argue that starting with who/what is providing the item's sentience makes a better start point for the item's abilities, personality, alignment, and even goal.
After all, there are a good many powerfully magical creatures in the various Monster Manuals to provide a source of magic.
Djinn, Efreet, Marid, and Dao provide elemental options and occasional wishes.
Celestials provide a myriad of powers, and a few might have deliberately chosen to be bound to the item. Alignment of wielder matters to these items.
Demons, Devils, Yugoloths, and other Fiends bring their own powers to bear, but like the Celestials bound, alignment matters, in that these items actively strive to bring the wielder's alignment to match the item's.
Dragons, Sphinxes, Kraken, Morkoths, and all the other beasties out there that have intelligence of their own, some degree of power, and preferably lair actions.
A wide variety of fey beings and assorted faeries could make for interesting spirits.
Expanding on the official options means asking who/what has possessed the item, who/what is the item haunted by, and what type of magic can create a fully sentient creature (then again, that is the dream of AI, correct?)
An item that is actually the phylactery of a lich (or a horcrux, ala' Harry Potter), provides an opportunity to redeem the lich or at least use its powers for good before it destroys you.
Hauntings might be the creator, one (or more) former wielders, beings slain or corrupted by the item or some combination of the above. Related to this may be the beings that were sacrificed in the creation of the item itself. A blade called Mocker that is imbued with the spirit of a polyglot sarcastic bard that regularly uses Vicious Mockery on opponents and wielder alike is a fine example, as is the sword named Glory, created from a glory-seeking fighter (or perhaps crusader) that forces the wielder to attack the biggest thing in sight.
Something fashioned entirely from magic would be beyond mortal ken, though not divinely (infernally?) inspired accident. This isn't to say such items don't exist, merely that mortals (like ALL PCs) won't be creating these items.
The 5e rules linked above state that "consumable items like potions and scrolls are never sentient." That is hogwash. Horsefeathers, if you prefer. A sentient consumable spends its time either desiring a suitably dramatic consumption or begging for its life, such as it is, to be spared. From the DM perspective, this can provide a wealth of entertainment.
Sentient items can be delightfully one-dimensional in personality and committed to their agendas. If ever in doubt as to how to portray an item, I lean on Telecanter's Creepy Commentary for inspiration.
With all this talk of sentient items, I should probably provide a few of my own. Here are three:
Hemp: Hemp is a sentient CN 50' coil of Rope that functions as a Rope of Climbing as well as a Rope of Entanglement. Hemp wants to be useful and regularly makes suggestions for use. If ignored, Hemp will uncoil and 'do it myself' in a huff, moving like a snake towards its objective. If Hemp feels unappreciated, it is not above slipping away in the night. If truly offended, it will strangle its owner in the night before slipping away.
The spirit of a Brownie is bound to the rope, the process shattering the fey's psyche in the process, but leaving its desire to be helpful. Aside from its Climbing and Entangling, the carrier of Hemp has a strong urge to seek out and consume honeyed milk.
Thirst: Thirst is a sentient LE dagger, technically an athame or maybe a kris, due to the wavy blade (when in a mood, Thirst will argue this point). Thirst enjoys the terror that killing defenseless sentient beings brings, and feeds on it, because a fiend is bound to the dagger.
The bound fiend is an Amnizu, and as such, it communicates solely through telepathy, and takes delight in compelling its wielder to commit vicious murders, through Dominate Person if smooth words fail to work. Its regular urgings (and subsequent murders) cause all sorts of complications.
Wielders benefit from innate knowledge of Infernal, Darkvision, an immunity to poison, and resistance to both fire and cold. When murdering a bound and conscious victim, the wielder is not only Healed as per the spell, but also gains temporary +1d4 to a random stat (S, D, C, I, W, or Ch). Yes, murdering several helpless beings allows for multiple stat boosts that can stack. These temporary boosts last until the next long rest.
What Thirst really wants is to be set free from the bindings. Destroying the dagger is the simplest means to do, but the bound Amnizu is incapable of telling anyone how exactly to destroy it.
Fortunato: Fortunato is a sentient NE ceramic theater mask of a bearded man wearing an odd helmet. An ancient bard is bound to this mask (due to well-deserved divine curse, but Fortunato is unlikely to share that, preferring a different story that paints him as the victim). Close inspection reveals the mask had been painted in the past, and Fortunato would love to be painted in full color again (being quite vain). When carried, Fortunato haunts the dreams and whispers in the mind of the carrier, urging them to don the mask. When worn, Fortunato confers all the skills, abilities, and spells of a 13th level bard with an 18 Charisma and can (and WILL) speak on its own. Ideally, Fortunato is able to dominate person the wearer and become the dominant member of the relationship. If Fortunato feels that attempts to dominate may not work, it instead urges to 'let me do this for you,' and strives to ingratiate itself with its usefulness.
If Fortunato successfully dominates its wearer, the wearer spends all ready wealth on a long bout of debauchery, carousing, and other forms of excessive hedonism - all while wearing Fortunato. Of course, when the wearer regains control (if the wearer regains control), they are the ones left to suffer the regrets and consequences of Fortunato's actions.
Note that Fortunato will force dominated wearers to attack bricklayers and anyone offering its wearer Amontillado to drink.
It occurs to me that two of the three items above are likely to get the boot once their agendas are discovered, so here is a fourth that PCs are more likely to keep around.
Astrophel: Astrophel is a sentient LG hand mirror with a
Planetar bound to it.
The bearer of Astrophel is immune to poison and charming, resists radiant damage, and saves at advantage against gaze attacks. When aligned with Astrophel's purpose, the wielder knows when it hears deliberate lies, and can see through illusions via True Seeing. In addition, Astrophel will Commune 1/day, Raise Dead 1/day upon anyone reflected in the mirror, and provide Healing Touch to anyone that concentrates while looking into the mirror.
Those that do not align with Astrophel's purpose (not necessarily alignment) only see their doom when they look into the mirror, as the Planetar truly knows the myriad ways the viewer is likely to die in any number of possible futures.
Astrophel's purpose is to throw down the unjust Tyrant King and his infernal allies.
Overall, sentient items bring lots to the table, and can be an interesting way to increase the capabilities of a single or few PC campaign without too many NPCs.
What sentient items have shown up in your games?