So, we ended the previous session of one of my adventures, We ended up picking a fight with a Beholder, (I don't know if anyone remembers me discussing it, but this is the same beholder I threw against the party at level 4 when I was the DM.) we are a level 10 party now, I'm finally a badass wizard. we will probably be 3, maybe 4 in size next session. And one of the questions that will inevitably arise is if Control Winds as a spell will have any impact whatsoever against the big floaty head? Specifically, I'm referring to the Downdraft ability of the spell, which RAI is probably designed as it is to ground flying enemies by knocking them prone. Rule as written, though, it won't knock the Beholder prone, because he is immune to that condition, but is it reasonable to infer that the beholder will be grounded by strong downward winds, or would the spell be useless against it?

What do you think? any opinions given would be valuable for preparing for next week's session!

"Spells officially do only what's described in their text. However, players often come up with creative, alternative uses, which DMs are responsible for adjudicating. I encourage a DM to say yes to such uses when they're non-disruptive and fun."

The spell does what it says it does, no more and no less. A Beholder's immune to the prone condition, and even if it could be knocked prone it still wouldn't fall because it can hover. Note that even a creature that can be knocked prone and fall would still be perfectly fine if it makes its Strength saving throw, so it'd be a bit strange to ground the Beholder, but you certainly could bend the rules if you think it'll make things more fun.

I'd direct the players towards the Earthbind spell if they're looking for a way to ground a Beholder.

Control Winds is a 5th level spell while Earthbind is a 2nd level spell.  I'd let it ground the beholder for one round if the beholder fails a strength saving throw with advantage and I'd  have the beholder use it's anti magic eye the next round to dispel it.  It's a creative use of the spell so it should have a chance of working, but a beholder's hover ability will give the beholder an advantage on the save.

Of course the party shouldn't get close enough to the beholder for grounding it to make a difference.  Beholder's have a genius level intelligence and they're physically weak.  They're smart enough to arrange things so that grounding them for a couple of rounds won't affect them that much.

Both valid points, though earthbind is a situational spell that we don't have on hand, and I wouldn't give a beholder advantage on the save if he can free himself from the effect as easily as you make it to be. Also, antimagic eye and AMF don't Dispel aoes, they just suppresses the parts of the aoe effects in the AMFs area, and that doesn't include the beholder's space, last time I checked. So assuming the spell works on him, the beholder is faced with an ultimatum: unless the beholder is constantly looking up with his central eye, he'll have to deal with the wind. It's either shut down a player(s) magic or fight at full force on the ground. That already makes a big difference to me. 

At face value it seems the spell should not work, since the beholder is immune to being prone. Not only that, but even if it did knock him prone, the flying movement rules state:
If a flying creature is knocked prone, has its speed reduced to 0, or is otherwise deprived of the ability to move, the creature falls, unless it has the ability to hover or it is being held aloft by magic.

So even then it would not fall since the beholder can hover. Heck, even if his flying speed is reduced to 0 and it was knocked prone he'd still be up there, just unable to move. Beholders don't really seem to fly with the use of air displacement or helium sacks or something, though that's entirely up to the DM. They are highly magical creatures with strange abilities, so I don't think that it would work. 

Even if it worked, I think that a beholder is smart enough either prevent it from happening via preparation (as Tim said above) to work out how to cancel it or deduce that the guy in the robe is probably the one responsible ergo *zap zap zap* there go three eye rays to break concentration and he's home free.