Can a familiar use a magic item to cast a concentration spell?

Question

As there are items that do not require attunement and allow the casting of spells through them, such as the wind fan, is it possible to have the familiar maintain concentration on this spell?

While holding this fan, you can use an action to cast the gust of wind spell (save DC 13) from it. Once used, the fan shouldn't be used again until the next dawn. Each time it is used again before then, it has a cumulative 20 percent chance of not working and tearing into useless, nonmagical tatters.

Note that this fan does not require attunement and is a totally stand-alone tool in your familiar's hands. An octopus, rat or weasel would arguably have the ability to hold and wave such an object around.

Answer 1

Nothing prevents your familiar from concentrating on a spell or using a magic item

The section on "Concentration" states the following:

Some spells require you to maintain concentration in order to keep their magic active. If you lose concentration, such a spell ends.

If a spell must be maintained with concentration, that fact appears in its Duration entry, and the spell specifies how long you can concentrate on it. You can end concentration at any time (no action required) [...]

It then goes on to discuss ways that concentration might end, so the above is all we need. The things worth noting is that nowhere does it limit what type of spellcasters or creatures can concentrate on a spell. Similar the ability to cast spells is not limited to certain creatures, nor is the ability to use magic items.

The fact that familiars can use magic items are supported in the following questions:

• "Can a familiar attune and wear or use a magic item?"
• "Can my familiar use a Ring of Spell Storing?"

Similarly, there simply isn't anything in the rules that limits what creatures can use items, or even attune to them, which is further supported by the following questions:

• "Can beasts benefit from magic items?"
• "Can an animal attune to a magic item?"

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All of the above questions have answers of "yes" because nothing prevents a creature from using a magic item, and similarly, nothing prevents a creature from concentrating on a spell. If your familiar uses a Wind Fan and casts gust of wind, they will concentrate on the spell.

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Finally, it is worth noting that your familiar cannot cast a spell that requires an attack roll, this is because familiars cannot attack. This scenario is gone over more specifically in the question "Is a spell that rolls attack an “attack” for familiars"