"Choose an unoccupied 5-foot cube of air that you can see within range. An elemental force that resembles a dust devil
appears in the cube and lasts for the spell’s duration.

Any creature that ends its turn within 5 feet of the dust devil must make a Strength saving throw."

Does that mean it doesn't do damage when cast only at the end of another creature's turn?

No it literally says they take half damage on a successful save

So.... this only works if they end their turn in it.  Even moving it onto someone doesn't work...

So this is a spell thats basically extremely hard to use?  I mean how often will an enemy willingly stop inside of one?  much less your own allies.  I   guess if there are dust etc, you could use it every round to block vision to an extent... but thats expensive premium for that.

This, like many similar spells, are used for crowd control. They don't want to end their turn within 5 feet of it, so they'll move to avoid it. The added bonus is that, with this spell, the caster can continue to move the effect around, further 'herding' enemies as needed.

It occupies a 5ft Cube but it affects anything within 5 foot of that cube, so it affects something closer to a 15x15 square. If it sucks up debris that becomes something closer to  20x20.

For those wanting to make this spell work, think of it like a personal-sized, moveable fog cloud. Use this for the cover instead of the damage. Unlike fog cloud, the enemies will be reluctant to enter it too. 

I think it's niche but not terrible. Without the dust part it is pretty bad though.

I had a player try this once and then dump it as useless.

You ought to be able to hit enemies with it like flaming sphere, or have it do damage at the start of their turn like spirit guardians.

Sand in the eye! Heh! You’ll never get my plans!

The fact that enemies can just run through this, so long as they don't end their turn within the area, and totally ignore its effects, kills this spell.

This spell is actually great for confusing enemies. Imagine this: the players are fighting a horde of goblins, with a magical booyagh goblin leading them. Should be easy stuff for them, but suddenly the booyagh summons a dust devil! Just how powerful is this guy? Naturally, one of the martial classes swings their blade at the dust devil (who has been heavily obscured by the loose dirt around them), but doesn't strike anything. Did they miss? Did it have some immunity they aren't aware of? At the end of their turn, suddenly they are smacked and pushed back. What creature feature is this?
Yes, the players may figure out that it's not a REAL dust devil pretty quickly, but at least one of them has wasted a turn on a single non-existent enemy when they could have killed two goblins. The same goes for the players casting this spell. They may be able to save themselves some anguish for a round or two, in addition to its crowd control capabilities.

What effect does Dust Devil have on Cloudkill? Can't find anything about this on the interwebs.

Quote from JacesAcolyte >>

This, like many similar spells, are used for crowd control. They don't want to end their turn within 5 feet of it, so they'll move to avoid it. The added bonus is that, with this spell, the caster can continue to move the effect around, further 'herding' enemies as needed.

 It also creates a 10-ft. radius sphere of dust that heavily obscures the area.  If in the right environment, you can create a temporary "wall" of debris that blocks line of sight.    

I would also rule that the dust devil would deal damage when it moves into an area, because of the force it generates when it moves.  That would make it like flaming sphere in terms of mechanics.         

How long can the dust devil actually last? Can it be used to watch over the party during a long/short rest?

I'm not saying the spell is bad, I just don't see any reason to take it over flaming sphere? Flaming sphere does 2d6 fire damage DEX save for half and it can deal damage if rammed into an enemy as a bonus action. This one needs at least something that makes it feel unique or more versatile in a different situation.

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For anyone thinking a dust devil is a creature, its just what a small tornado is called. it does not create a creature called a "dust devil"