Could you negate a Beholder's antimagic cone by covering up its eye?

Question

e.g. throw a heavy blanket over it, or force it to close its eye somehow.

https://www.dndbeyond.com/monsters/beholder

The beholder's central eye creates an area of antimagic, as in the antimagic field spell, in a 150-foot-cone.

...and the antimagic field spell only describes it as creating an "invisible sphere", with no suggestion that it would be blocked by walls etc

However there is no language in the spell to suggest it would be exempt from the normal rules for "area of effect" spells: https://www.dndbeyond.com/sources/basic-rules/spellcasting#AreasofEffect

A spell's effect expands in straight lines from the point of origin. If no unblocked straight line extends from the point of origin to a location within the area of effect, that location isn't included in the spell's area. To block one of these imaginary lines, an obstruction must provide total cover.

If we consider the eye itself to be the "point of origin" of the spell then maybe covering up the eye would block the effect?

There is an answer here that comes to the same conclusion, that antimagic field/cone is blocked by total cover ("lead shield") https://rpg.stackexchange.com/a/116809/56593

OTOH it feels like covering up the eye is rather stretching the definition of cover.

Weirdly the rules for "total cover" seem to contradict those for "area of effect" above though: https://www.dndbeyond.com/sources/basic-rules/combat#TotalCover

A target with total cover can't be targeted directly by an attack or a spell, although some spells can reach such a target by including it in an area of effect. A target has total cover if it is completely concealed by an obstacle.

I guess maybe "some spells" means only certain spells which mention a specific exemption to being blocked by total cover.

Does the eye have to be exposed for it to work? Or does the cone project through mundane barriers?

Answer 1

It's a bit unclear; but this probably works

Something to note right away is that the rules about areas of effects are about spells; I would certainly assume these rules apply to area of effects from monsters for things that are not spells but that is my choice as a GM.

In this case, we do know the following (emphasis mine):

[...] The beholder's central eye creates an area of antimagic, as in the antimagic field spell, in a 150-foot cone. [...]

But unfortunately, this doesn't mean that what the Beholder is doing is a spell nor that it follows all the rules for spells. If a GM rules that the Beholder's Antimagic Cone ability does require an unblocked path to target somebody then yes, covering a Beholder's eye (by whatever means available) would prevent its Antimagic Cone from affecting creatures. But there's an "if" in there; and the GM does not have to come to this conclusion.

Personally, I have always treated non-spell Areas of Effect as having to follow the rules for Areas of Effect found within the Spellcasting section of the Player's Handbook. As such, I would allow a party to end the Antimagic Cone if they found a way to cover up the Beholder's eye.

Answer 2

Basically, no.

Consider the rules on cover which are on this page.

Something simple like a blanket does not qualify as cover so tossing one over a beholder won't have an effect.

If it did then, by the same logic, throwing a blanket over yourself would also block the antimagic effect.

Even total cover states that it only protects against being targeted by attacks or spells but that some spells can still reach someone behind cover if the area is included inside of the area of effect.

Even the spell antimagic field says nothing about cover or obstacle blocking the effect and this is the same effect in cone shape.