One of my PCs, a Wizard, found an item of demonic origins. Basically, the carrier develops a sort of connection with a dark entity that affects his personality over time and possibly provides demonic powers.
Another PC cast Detect Magic on the Wizard. I was not sure whether the demonic item would be detected. More generally, I am not sure whether certain creatures/items (dragons, demons, devils, celestials and so on) would be considered as magical in a standard D&D setting.
So, here is the question: can supernatural phenomena (creatures / powers provided by items) always be related to magic?
Best Answer
There is leeway to interpret Detect Magic so that it detects things other than spells and enchantments. The phrasing of the spell in D&D 5E says that it can detect
and this is open to interpretation, around the word "bears" which is not a games rule term used elsewhere. There has even been discussion on this site about whether memorised spells count. However, the consensus in groups I have played in (and I think a widely held one) is that Detect Magic is equivalent to "Detect Spells, Spell-like Effects and Enchantments". That is, it will detect spells in progress, spell-like powers of monsters, and permanent enchantments on scrolls, and standard magic weapons. Under that interpretation, will not detect supernatural powers, such as a druid's Wild Shape, or the fact that something is demonic. Detect Magic also will often detect special effects associated with locations with arcane themes in adventures, whether official or homebrew.
Demons, celestials, elementals, and fey are detected by Detect Evil and Good (as it happens, dragons are not). If they were already detected by Detect Magic, then Detect Evil and Good would much less useful. So that is good reason to suppose that a demon that was not currently casting a spell would not be detected by Detect Magic.
So, no Detect Magic worries for your custom item then? Maybe. I think you need to consider the effect on your game and story.
The main issue is player expectations. An "item of demonic origins" would often be thought of as a regular magic item by players, if it creates a mind effect with no obvious physical cause. There are examples of demonic-looking and demon-themed magic items in the rules. The thinking might go "That's a magic item, nothing makes it different from any other magic item, and they can all be detected, right?" So they could expect the item to be detected like any other magic item. Switching this expectation on the players will need consistent logic, and ideally there should be hints dropped into the story on the item's special nature, during or before the reveal that the PCs behaviour has been influenced. You don't want your players to feel cheated, or that you'd played some rules-lawyering "gotcha" on them.
For example, you might consider that your item is still "alive" and counts as a demon using a supernatural power for purposes of Detect Evil and Good. That could be an interesting twist, and still leaves door open for players to discover the item's purpose and deal with it in character, using 1st level magic. If you can drop clues into the story about these kind of items and why they are different, even better, as it reduces the risk of players feeling cheated, especially if they get the clue later and try something.
If you want to make the item tougher to detect, then perhaps it has had Nystul's Magic Aura cast on it to confuse divination spells. Again you might want to leave clues this has happened. A nice clue would be if the item belonged to some evil wizard that he team has defeated or otherwise got hold of his/her spellbook, then Nystul's Magic Aura is there plain to see. Players often won't get the link when they find such clues, but they will figure it out during a reveal (or you can even explain it to them), and in my experience this can turn a "gotcha" moment into "so that's how it was done!" revelation that makes the story hang together even better.
Also, you should think through now, before more attempts are made to understand the item, what other divination spells could reveal about the item. That's because you want to be driven by consistency on how the item behaves more than by the goal of having the item take effect on the hapless PC wizard.