So this combo seems straight forward, but I never see it talked about. There are a number of creative ways to deal with flying monsters. But this one occurred to me, and I never see it discussed. No saves, so no need to worry about high saves nor even legendary resistances, no strength checks, nothing. It just happens.

Am I missing something here?

So, you're a level 5+ Hexblade Warlock.

Step 1: Cast Fly (or have someone cast it on you)

Step 2: Fly up to the creature.

Step 3: Hit the creature with your stick (weapon of choice) and Eldritch Smite it.

Eldritch Smite

Prerequisite: 5th level, Pact of the Blade feature

Once per turn when you hit a creature with your pact weapon, you can expend a warlock spell slot to deal an extra 1d8 force damage to the target, plus another 1d8 per level of the spell slot, and you can knock the target prone if it is Huge or smaller.

Alternate.

Step 1: If you have the Improved Pact Weapon Invocation, just shoot it with a longbow and add Eldritch Smite.

That's it! It falls out of the sky taking weapon damage, smite damage, and fall damage. No strength or dex checks, no saving throws, nothing. It just happens.

Am I missing something?

When you build character around a single purpose (making flying things fall), you ought to be good at that single purpose.

There's actually nothing in the rules about a flying creature falling out of the sky if they're knocked prone

Prone just means they can only "crawl" for movement, and "crawling" just says each foot of their movement costs an extra foot. So they would just fly slower for a bit until they righted themselves

To make them fall, you'd need something that reduced their speed to 0

Never mind, I found the rule stating exactly that. The search function on this site is a mess

It's still not 100 percent foolproof though, depending on how the creature is flying:

Flying Movement

Flying creatures enjoy many benefits of mobility, but they must also deal with the danger of falling. 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, such as by the fly spell.

Quote from TexasDevin >>

When you build character around a single purpose (making flying things fall), you ought to be good at that single purpose.

I'm...not sure this "build" is "designed around a single purpose" this is just one character option he's talking about, not some complicated combo. That isn't really a "build" or even a specialized one, for that matter.

We're really just talking about the Eldritch Smite ability in a vacuum.

But OP, you're not missing anything. This ability is very strong and doesn't get talked about as much as it should for its anti-air capability.

Quote from TexasDevin >>

When you build character around a single purpose (making flying things fall), you ought to be good at that single purpose.

I agree, but the nice thing about this is it require nothing special, no compromises, and no build tweaks. It's just a Hexblade Warlock, and you're almost certainly going to want the Eldritch Smite anyway.

Being able to knock something huge prone at will like that has such a cool factor to it :)

Quote from AntonSirius >>

There's actually nothing in the rules about a flying creature falling out of the sky if they're knocked prone

Prone just means they can only "crawl" for movement, and "crawling" just says each foot of their movement costs an extra foot. So they would just fly slower for a bit until they righted themselves

To make them fall, you'd need something that reduced their speed to 0

Never mind, I found the rule stating exactly that. The search function on this site is a mess

It's still not 100 percent foolproof though, depending on how the creature is flying:

Flying Movement

Flying creatures enjoy many benefits of mobility, but they must also deal with the danger of falling. 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, such as by the fly spell.

Ah, this seems to be the catch. I always knew if a creature's speed is reduced to 0 it stays afloat if it has hover. I just thought the same was not true for being knocked prone. Most of the stronger flying creatures have hover.

I often think that rules and systems to deal with flying could have their own book. 

Having said that I often have trouble with automatic things happening without a save of some kind and wonder if the writer has thought of all the implications of the rule they put into place. 

MDC 

Eldritch Smite is great for poaching flying threats. But you don't need fly. Just shoot a bow since the Eldritch variety isn't limited to melee attacks like Divine Smites are. Hexbow is quite a strong build with a lot going for it.

Hmm.

The main issue you have with this combo is that it's seemingly too resource intensive for very little pay off, and it takes too long to set up.

You can't assume that you'll have some one else cast fly on you, so you'll have to assume a turn of setting up. Next turn, you potentially fly 60 ft to get in melee range of the target, hit, and smite. 

The end result, at best, is you get your weapon damage, the smite damage, and 6d6. If we do this at 5th lvl, the soonest this can be done, then it's 4d8+6d6, or 39 damage on average not including weapon and other possible riders. 

You just spent both of your slots and two turns to do 39 damage. I don't think that's worth it.

That's exactly why the move is to use a bow

Quote from cgarciao >>

Hmm.

The main issue you have with this combo is that it's seemingly too resource intensive for very little pay off, and it takes too long to set up.

You can't assume that you'll have some one else cast fly on you, so you'll have to assume a turn of setting up. Next turn, you potentially fly 60 ft to get in melee range of the target, hit, and smite. 

The end result, at best, is you get your weapon damage, the smite damage, and 6d6. If we do this at 5th lvl, the soonest this can be done, then it's 4d8+6d6, or 39 damage on average not including weapon and other possible riders. 

You just spent both of your slots and two turns to do 39 damage. I don't think that's worth it.

As HeironymousZot points out, if you use a bow, then you don't have to cast Fly, at all.  You can knock a flying creature prone from the ground and watch them fall.  

Yes but then you're a Hexblade with a bow, which is objectively worse than just using Eldritch Blast and leads you to wonder why you're even a Hexblade in the first place. Also, as far as I know Eldritch Smite is much less of a "you'd take this anyway" type of invocation than is being implied here. There are a lot of good invocations, and this one often doesn't make the cut.

As TD said earlier, you're really not likely to have this combo unless you're specifically building for it. That's why it's not mentioned much.

Sharpshooter has something to say to you about being objectively worse than using Eldritch Blast. Hexbows are nasty.

Quote from scatterbraind >>

Yes but then you're a Hexblade with a bow, which is objectively worse than just using Eldritch Blast and leads you to wonder why you're even a Hexblade in the first place.

I mean I can tell you that one of the cool character types I wanted to do with Hexblade is the Weapon Morpher trope, so I think it's a valid desire to be able to manifest even ranged weapons, which is why it's nice that Improved Pact Weapon allows bows and such. I also usually ignore the advice that it makes a bow, but not arrows.

Quote from scatterbraind >>

Yes but then you're a Hexblade with a bow, which is objectively worse than just using Eldritch Blast and leads you to wonder why you're even a Hexblade in the first place. Also, as far as I know Eldritch Smite is much less of a "you'd take this anyway" type of invocation than is being implied here. There are a lot of good invocations, and this one often doesn't make the cut.

As TD said earlier, you're really not likely to have this combo unless you're specifically building for it. That's why it's not mentioned much.

 It is not objectively worse.  Eldritch Blast is a D10, and a Longbow is just a D8, but a Longbow can be boosted by Sharpshooter, or by using a magical weapon, whereas Eldritch Blast doesn't get either of those.

In a game with no magical gear and no feats, Eldritch Blast is better than a Hexbow.  But I don't know of anyone that runs their games that way.

Quote from gabrielrockman >>

In a game with no magical gear and no feats, Eldritch Blast is better than a Hexbow.  But I don't know of anyone that runs their games that way.

In a game with no magical gear a Hexbow could be one of the only characters with a +1 weapon from the Improved Pact Weapon. Making the conjured pact weapon arguably even better in that particular scenario.