From the Quest quest introduced in the Visions in the Darkness event.

File:Banderling Aggressor Backpack Live.jpg
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Banderling Aggressor Backpack
Value: 250
1 Burden Units
This backpack, sewn from the scalp of a Banderling Aggressor provides comfort as well as carrying space. Use this item to close it.

Notes

You give Collector Aggressor Token.
Collector tells you, "You're in luck. I was able to procure a few of these backpacks from the tailor. Carry it with pride!"
Collector gives you Banderling Aggressor Backpack.