Sentinel Raven uses too much awesome magic

Question

The Raven Queen Warlock in my group got a Sentinel Raven, as per the Lvl1 Class Feature. This raven is now attuned to a magic ring and wields a magic wand. The wand is not that big of a problem, but the ring allows the raven to cast Earthen Grasp and Wall of Stone, amongst others. (Both are homebrew items.) This has led so far to some pretty awesome moments, but also to problems. That's because this Sentinel Raven cannot be targeted by harmful effects or spells and can't take damage while perched on the warlock's shoulder.

While perched on your shoulder, the raven can’t be targeted by any attack or other harmful effect; only you can cast spells on it; it can’t take damage; and it is incapacitated.

So it just flies off the shoulder, does some magic, then lands back on the shoulder in the same turn. This makes it (virtually) impossible for me as the GM to break concentration on spells like Earthen Grasp. That's really annoying sometimes and it makes balancing encounters rather difficult.

Previous answers seem to agree that familiars can indeed attune to and use magic items (Can a familiar attune and wear or use a magic item?). Plus, the magic items in question can be used by a spirit that assumes the form of a raven, as they don't have any constraints that would restrict their usage and the spells cast from them do not require any components.

Is this an intended feature of the Sentinel Raven? Am I overlooking something? How can I solve this problem in the game and/or at the table?

The warlock player really likes the raven and is a huge fan of the versatile magic it can do now with the magic items he gave it. I don't want to punish anyone, just find out if this is legit and what to do if it is not.

Answer 1

Yes, you are missing something; an incapacitated creature cannot maintain concentration:

Being incapacitated or killed. You lose concentration on a spell if you are incapacitated or if you die. (PHB 203)

Once the Raven lands on the Warlock's shoulder, Earthen Grasp ends as it becomes incapacitated.

While perched on your shoulder, the raven can’t be targeted by any attack or other harmful effect; only you can cast spells on it; it can’t take damage; and it is incapacitated.

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The conversation with the Warlock player

Hey Bob, I previously allowed your Raven familiar cast Earthen Grasp the way it does with the magic items. It was fun for some time but I found it was too strong an interaction. So I dug around to see if I ruled correctly, and it turns out your raven cannot keep concentration while on your shoulder. It can still fly around while concentrating but it can now take damage.

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Obligatory warning about UA. Unearthed Arcana is playtest material, just as the UA content requires a DM's nod, it also requires the DM to balance the content in their game when they see something too powerful, so you are well within your power to disallow any weird interactions with UA. You might even want to send feedback to WotC about it.

Answer 2

daze413's answer covers the raven side of your question well, but there's another issue that needs to be touched upon: the homebrew magic items.

Most magic items that grant the ability to cast powerful offensive spells require attunement by a spellcaster. This prevents abuse by limiting powerful spells to the few characters that are already capable of casting them; it'd be difficult to balance encounters if every single player (and their pets, mounts, and NPC hirelings) could cast high level wizard spells just by getting their hands on the right magic items.

You should consider limiting who can attune to your homebrew ring and any future homebrew items you design that provide spellcasting.

Answer 3

Use Ready action to initiate when "the raven begins to cast a spell" or "when the raven takes flight" and let the bad guys pound on it prior to / during spell casting. With the game stats of a raven, it likely won't last long.