Some info (mostly speculation) is available in this article : From Dunne’s comments and those schematics, this looks to be a spear-like weapon that can unscrew into being two short-range stabbing weapons as well.
Essentially, a double-ended spear that can also act as two daggers.
One tip is definitely dragonglass—we can tell that from Arya’s schematics.
Is the other end using her Valyrian steel dagger blade?
Is that flame?
Just an ornate handle?
Unlikely just a handle—that doesn’t seem like Arya’s style.
We’ve actually probably already seen Arya using this weapon in a few brief shots from the Season 8 trailer.
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Tommy Dunne, the weapons master for Game of Thrones , has the following to say on what is almost certainly the weapon Arya asks Gendry to make:
In a recent interview with Vanity Fair’s “Still Watching” podcast, Thrones weapons master Tommy Dunne teased what is very likely this new blade: “There is one creation in Season 8 that is absolutely phenomenal.
It’ll be a showstopper.
That was great to finish.
It was great to make, great to design in that it was able to be reverse-engineered and made into two different items.”
Vanity Fair, Game of Thrones:
Gendry Is Making Arya a “Showstopper” Weapon As of now we don't know what it actually is, and all we have to go off is the schematic and Dunne's comments.
That said the schematic shows two pieces that fit together and can come apart with one end being tipped in dragonglass so it is likely a long weapon that can be split into two dagger like weapons.
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Everybody seems to assume it's a spear - it's not, primarily, and she doesn't seem to use it as such most of the time.
Of course, it's sharp at both ends so you can poke it at people [
using dragon glass so would be effective against wights as well as humans], but that's not really how we see her use it.
Her weapon is basically the stick she learned to use so well fighting the waif in Bravos - with two pointy ends to stick them with as well.
This is all her years of training being put to use.
She used it to great effect on top of Winterfell's walls...
She split it in half when she first moved indoors, into more restricted space, but mainly she still wielded it like her stick.
I'd say ninja-style, even though that's not a discipline that exists in this world.
Why it was splittable
I'm not really sure about.
She split it when she went indoors, very briefly, but the next time we see her she's re-assembled it.
I checked some timings...
She actually loses the 'posh stick' as she dives over some wights down a flight of stairs, and is not seen again with it.
[cross-ref to https://movies.stackexchange.com/a/100243/25773 with gif]
She doesn't have it when she jumps off the parapet onto the sloped roof.
When we next see her again indoors, she picks up a dragon glass dagger from the floor behind some fallen furniture.
She keeps this right through the library scene & the chase afterwards.
She loses that when she's rescued by Dondarrion.
So she splits the stick for an entire 10 seconds & that's it.
All that for 10 seconds
;-)
She has the stick from 39:40, splits it at 40:00
She goes out of shot with it still split at 40:11 & the next time we see her
it's back together again at 40:18.
I first thought it split to be a long stick & a dagger, but it does split into two unequal sizes.
Splitting to a dagger didn't quite make sense to me, as she already was known to be carrying a Valyrian steel dagger; the one originally used on the attempt on Bran's life... ...and used, rather appropriately in the end, to kill the Night King and save Bran.
From comments -
In the godswood she doesn't have the stick at all [see timings above]
she has only the Valyrian steel dagger, which she drops from one hand and catches in the other.
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While i cannot be certain, it appears it may be an Atl-atl or a Spear
Thrower
The user holders on end using a lever or rope to then propel the spear faster and further.
the spear itself is shorter and expendable so
perhaps that is what she wants.
the design doesn't look quite right for one but it looks similar at least I'll be honest its a bit of a stretch
but it was what first came to my mind when i saw it
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It's a Dragonglass Spear according to the HBO - Making Game of Thrones website.
As of S08E03, "The Long Night", it is shown to be a spear with a dragonglass head at both ends and can be separated in the middle to be two shorter spears.
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In season 8 episode 1, Arya asks Gendry to forge her a dragonglass weapon (which we can see drawn).  What is the purpose of this weapon?
What is Arya's weapon design?
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Arya's weapon in GOT season 8 episode 1 is essentially, a double-ended spear that can also act as two daggers,one tip is definitely dragonglass. Essentially, a double-ended spear that can also act as two daggers. One tip is definitely dragonglass.
Some info (mostly speculation) available in  this article. Essentially, a double-ended spear that can also act as two daggers. One tip is definitely dragonglass.