In the campaign I'm running, one of the villains is a cursed artifact that could (and did) take control of its user. For an easier time on explaining my problem with those mechanics, I'll simply use the Eye of Vecna here since it has very similar properties.
So according to the description of the Eye of Vecna, it has a number of charges that can be used to cast spells, but also states the following rule about using said charges (Dungeon Master's Guide, p. 224):
Each time you cast a spell from the eye, there is a 5 percent chance that Vecna tears your soul your body, devours it, and then takes control of the body like a puppet. If that happens, you become an NPC under the DM's control.
However the descriptions of this and other items capable of possession don't really seem to elaborate on what happens to the item itself after its conscience has taken control of the being attuned to it, and the only other instance outside of a sentient item's description regarding possession I found is in the DMG regarding conflicts between the user and the sentient item (item can try to take control, saving throws are rolled etc.).
Is there anything in the rules as written detailing whether a cursed artifact that succeeds at possessing the user who is attuned to it can still use its own properties normally?
For instance, would the Eye of Vecna (as in Vecna's consciousness) still have access to all of its artifact properties as if it were just a normal user (minus the risk of possession)?
Best Answer
A sentient magic item always controls its own abilities
The section of the DMG (p. 214) on sentient magic items (and the corresponding part of the basic rules) contains a passage relevant to your question:
So even if the item takes control of or possesses its wielder, it doesn't matter whether or not that possessed wielder can still access the item's abilities, because the wielder was never truly in control of the item's abilities to begin with. The item already had control of its abilities, and it maintains that control after possessing its wielder.
The Eye of Vecna is not a sentient item
Note carefully the wording of who possesses the user of the Eye of Vecna (DMG, p. 224; emphasis added):
The Eye of Vecna is not itself a sentient item. It is merely the eye of the deity named Vecna, and this deity, not the eye, is what possesses the eye's hapless user. There is no reason that Vecna would not be able to use his own eye after possessing the newly soulless body of its user. One has to assume he has prior experience with it, after all.
Your homebrew artifact works however you say it does
Obviously, the reasoning I've given above for the Eye of Vecna is very specific to the nature of that artifact. But that is always going to be the case for artifacts. Artifacts are not just ordinary magic items that happen to be super rare and super powerful. They are a special category of magic item, distinct from all others, and furthermore each artifact is unique, as mentioned in the DMG's section on artifacts (p. 219; emphasis added):
The point is, every artifact is different from every other artifact. Just because the Eye of Vecna works as I've described doesn't mean your artifact has to work that way. For example, you could have a circlet containing a gem that houses the soul of a powerful archmage, and when it possesses its user the circlet disappears and the gem permanently embeds itself it the user's forehead, destroying the item and effectively replacing the user with the archmage. Or it could be a sentient item that simply takes command of its user (similar to a Dominate Person spell) without killing them or destroying their soul, while remaining a distinct item that the user can continue using as normal (at the item's command, of course).
Remember, an artifact is a plot device first and a giant block of rules text second. So first think about what makes sense for the nature of the item as you've envisioned it, and then write the rules text to match it. (But if you think this is something likely to fall into the PCs' hands, you should consider having a way to undo the possession.)