I have this idea: a goblin warchief who somehow got a hold of a beholder's central eye, which is attached to the center of his shield as an ornament. He can activate the eye for a reaction to counter ONE magic ability from a player, and if the players are grouped up to where the eye gazes at more of them, the eye also prevents them from casting spells.

How powerful is this? For a level-6 boss fight, are there any additions I should make beyond just "give goblin boss the eye of the beholder, but keep all other stats the same"?

The big concern I would have is with the PCs getting control of that item. The main effect it has is that it will prevent spellcasters from AoEing the minions to death, so it depends on how the rest of the fight is set up.

If you're worried about the PCs getting their hands on it and using it, you could make it require attunement by some characteristic they do not have. For a good party, "requires alignment by an evil character." Goblin chief's evil, he can attune. Players are non-evil, they cannot. This would probably lead to them trying to destroy it so it doesn't fall into the wrong hands again, but they won't be able to use it.

If you’re worried about our options as to how balanced the whole encounter will be for the party, I have absatively no way of assessing that accurately. As far as I can tell from your description, it’s just the one Boss Goblin with an eyestalk stuck on a shield against a party of anywhere between 2 to 12 6th level PCs. If that’s really the encounter, that Goblin may as well be a corpse when they find it.

If you’re wondering if we think that Magic item would be balanced once it fell into the party’s hands, then no, I do not. Here are my reasons:

• Your item is simultaneously a shield, and Magic wand of “Counterfeature.” That is a single Magic item acting as both a reasonably indestructible shield, and a magic wand, and the bearer never has to let go of either leaving the other hand completely free for whatever. Without the action economy to regulate some of their privacy. If for no other reason then it’s your wife’s place tonight.
• There are a number of ways in 5e that a character can interfere with the Spellcasting attempts of monsters, NPCs, and characters. I cannot think of anything official of the top of my head that is accessible by PCs that allows them to interfere with the “magical abilities” of other creatures. And, wile I know for a fact that there are rules covering what is and is not considered “magical” in relation to class features, that still leaves a grey area. Consider that this item could be used against an Arcane Archer Fighter, completely shutting down one of the two precious Arcane Shots they get per short rest. That would make this a very rare and potentially very valuable Magic item.
• And the way you described things, this item can be used an indefinite number of time, and only costs a single reaction to use. It also seems that, since the potential effects from this item include that it “prevents them from casting spells” which indicates a minimum duration of at least one round. It also implies that the effect persists even after the target has moved out of that area.

So, I think for me, if I were gonna write a thing like this, I would:

• Make it a “regulation” wand without it being attached to a shield at all, or perhaps maybe at most having the wand sheath attached to the shield for quickdraw circumstances. That’s still a really useful thing to have, but still not as bad as simultaneously wielding two items for the price of one. I would probably have the reaction only affect a single magical feature or trait, specifically the one that triggers that reaction, and it should require some limitations on its number of uses. I would also require some sort of roll, perhaps an Arcana check or something to see if it works, where the DC is dictated by character level or something.
• I would also style the second ability that is capable of jamming up all Spellcasting of up to two characters for a whole round to require a full action, I would word the targeting section to read more like Acid Splash, and I would also require either a check by the wielder, or a save by the target(s).

That would still be a very strong Magic Item IMO, but I may be wrong.

I’m sorry, I’m super tired, I hope that was all more legible to you than it was for me just now.

Edit: I fixed some stuff, and some stuff I had no idea what I intended to say so I just scrubbed it. It turns out that it was in fact not as legible as I had hoped.

It's the eye off a dead thing. Every day it becomes more putrefied and the range of effect is reduced by 10 feet.