Game of Thrones final season [spoilers]

[Amarok]

I think Arya will go too far to disguise herself as an undead walker to get close and end up actually becoming one then have to be killed by her protector the Hound

I hope not

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A wooden stick with a glass tip? Yeah, I'm not seeing it. If all you need to kill an undead dragon is to scratch it with some obsidian, then I imagine a few arrows would do the job much more quickly, safely, and efficiently.
I'm sure the spear-thing will have some important part to play, since the show really wants us to pay attention to it. But regardless - it makes sense that Arya would get a battle weapon like that, because:
A. her regular weapons are more suited for stealth assassination (dagger), or close quarter duels ("Needle"), than for a full-on battle,
B. she's small, so she could use a range boost against white walkers on horseback,
C. she was seen training with a staff (or rather - being repeatedly beaten with one) at the House of Black and White, a couple of seasons ago, so it's a good way to tie things together and make that whole plot seem like less of a waste of time.

Well, we know obsidian is useful against the undead. That's why they are trying to make everything from it, after all. We also know that spears like this are effective against dragons for the simple reason that this particular dragon was brought down by a spear.

There's also poetic justice in one of the smaller members taking care of the large undead dragon.

You do have some points about extending range, though.

To be fair that was a magical spear forged by the Night King . Thou I suppose a spear of dragon glass might kill it .