If a monster with innate invisibility (for example an invisible stalker), picked up an item, would the item remain visible? I’m asking because an invisible stalker can be created to retrieve an item. I was planning for the BBEG to send a few after the magic macguffin, and I was wondering if a stalker managed to take it, would the players be able to see the item in its hand?

Effects that make held/worn items invisible say so.

The invisible stalker is an air elemental. It is invisible because it is made of air, not because of magic.

TLDR: the you can see items it holds, yes.

Quote from DxJxC >>

Effects that make held/worn items invisible say so.

The invisible stalker is an air elemental. It is invisible because it is made of air, not because of magic.

TLDR: the you can see items it holds, yes.

 Thanks!

This is a DM thing. There is nothing about the invisible condition stating your items or clothing or things you carry become invisible with you. The invisibility spell specifically mentions that you become invisible and then goes on to add that anything the target is wearing or carrying also becomes invisible. Cloak of Invisibility, Potion of Invisibility and Ring of Invisibility also have this similar wording. This necessity of stating it implies things that become invisible by others means only have them become invisible and not their clothes and carried items. Which is fine for the Invisible Stalker because it's naturally naked anyway. I think this is a design intent for such creatures so they aren't completely perfect thieves. 

However, the spanner in the works are things like the racial choice of the Firbolg's Hidden Step ability. It has no wording like the above and just says you become invisible. So a strict comparative reading may suggest the firbolg goes invisible but their clothes and items don't, making it effectively useless, logically speaking. This is because when it came to making the firbolg as a race they use idiomatic speech, rather than rules jargon. They leave it up to you to have the common sense to understand the whole firbolg goes inviz - kit and kaboodle 'n' all. 

So, it's up to you. Do you apply the firbolg approach is the standard or take it as an exception? Bear in mind that if you choose for it to be the standard (and thus all stalky gets to be the bestest of thieves) your characters may eventually be able to use this for thievings also. Conjure Elemental spell is player accessible, as is the Planar Binding spell. So be wary of what tools you grant your players. 

Personally, I'd make firbolg one an exception and find a different way for the Bad Guy to obtain the MacGuffins - preferably in a way that the players actually have a chance to notice it and defend against it (which they wouldn't with the improved thief-stalky). If you want the Bad Guy to get the guffs without players being able to stop it then maybe a DM-Ex-Machina option is better than a By-The-Rules option is best.

TL;DR: it's up to the DM to interpret.

Quote from Cyb3rM1nd>>

This is a DM thing. There is nothing about the invisible condition stating your items or clothing or things you carry become invisible with you. The invisibility spell specifically mentions that you become invisible and then goes on to add that anything the target is wearing or carrying also becomes invisible. Cloak of Invisibility, Potion of Invisibility and Ring of Invisibility also have this similar wording. This necessity of stating it implies things that become invisible by others means only have them become invisible and not their clothes and carried items. Which is fine for the Invisible Stalker because it's naturally naked anyway. I think this is a design intent for such creatures so they aren't completely perfect thieves. 

However, the spanner in the works are things like the racial choice of the Firbolg's Hidden Step ability. It has no wording like the above and just says you become invisible. So a strict comparative reading may suggest the firbolg goes invisible but their clothes and items don't, making it effectively useless, logically speaking. This is because when it came to making the firbolg as a race they use idiomatic speech, rather than rules jargon. They leave it up to you to have the common sense to understand the whole firbolg goes inviz - kit and kaboodle 'n' all. 

So, it's up to you. Do you apply the firbolg approach is the standard or take it as an exception? Bear in mind that if you choose for it to be the standard (and thus all stalky gets to be the bestest of thieves) your characters may eventually be able to use this for thievings also. Conjure Elemental spell is player accessible, as is the Planar Binding spell. So be wary of what tools you grant your players. 

Personally, I'd make firbolg one an exception and find a different way for the Bad Guy to obtain the MacGuffins - preferably in a way that the players actually have a chance to notice it and defend against it (which they wouldn't with the improved thief-stalky). If you want the Bad Guy to get the guffs without players being able to stop it then maybe a DM-Ex-Machina option is better than a By-The-Rules option is best.

TL;DR: it's up to the DM to interpret.

Thank you!

The goal of the thievery is not to give the MacGuffin to the BBEG- actually the PCs just stole it from them. The BBEG doesn’t know who took it, so it makes sense that they (being a druid) would use conjure elemental to summon the stalkers and tell them to retrieve the item. Hopefully, and most likely, the PCs will win, but they’ll know that the enemy wasn’t entirely fooled. If the enemy does get it, I’ll figure it out. 

Based on that, I’ll go with the option of treating the firblog as the exception.