@Avalon
By commanding a minion it is impossible to do something like this, indeed. But if you have a claim on an artifact, you can demand it vie regular interaction - use a strong dominance hook to enforce it.

When you use magical triangulation and track down an artifact that your minion owns, you can then demand it via an event (this probably doesn't require claims).

But the easiest thing to do in such a situation is to simply imprison the owner of the item, as long as you have the first Arcana dynasty legacy perk. Here you don't need a claim either, and you have a chance to capture the item. Imprisoning a minion probably won't be difficult ;)

True. Valid point. But its just so many extra steps involved. Id imagine mind controlling a dude would allow me to take his shinies :D

Well, i captured someone who became the owner of an Artifact after she was already captured. Now it seems like i cant take away that artifact. Is it bugged?

If you have imprisoned a character and the capture of the artifact has not taken place, there are several possibilities:
1) You do not have the first Arcana dynasty perk.
2) The imprisoned character did not have an item that is classed as a magical artifact (some artifacts are special but are not considered magical e.g. protective amulets, Yaga's Broom).
3) The character may have managed to hide the artifact before being imprisoned, this also happens, there is a random chance of this happening. If this happens, the artifact is transferred to another person, usually a sibling or child - and you have to try again. These mechanics make this method of obtaining magical artifacts unreliable, and is meant to encourage you to get a claim on an item though ;) (otherwise it would be universally the best way to acquire magic items, as long as you are able to kidnap characters).

Generally, acquiring magical artifacts is difficult, and complications can happen. As a general rule, you should get claims on an artifact to make the process easier.

Hm, if you have the first Arcana dynasty perk, and have imprisoned a character who still has a magical artifact in their inventory - then yes, such a scenario is strange and probably shouldn't happen.

yes i have the perk. I imprisoned the heir of someone, who had the crown. After having this person in my prison, the liege died and transferred the item to my prisoner (she has it equipped). I cant use the option to take it away from her.

@Avalon
The capture of the artifact occurs at that point when the character is imprisoned. If the character does not have the artifact with them at that particular moment, you will not capture it ;) Admittedly, the situation is a bit strange, since the character later inherits the artifact (while in the dungeon), but ck uses some simplifications here.

The exploit that comes to mind for me is to release the character (with recruitment to the court) and imprison him again (since he already has the item with him).

Well, and as I said, if you have a claim on the artifact, you can directly demand the item.
Essentially, CK is done in such a way that in order to demand/obtain something, you need a claim on it. Sometimes it is possible to get something without a claim, but this is more by chance or through favourable circumstances.