This is story related and not missable. Mild spoiler alert
In order to obtain you must defeat the Matriarch, which is basically your classic berserker boss fight.
Here’s some tips to make the fight easier:
The trick to making this easy is to keep dropping the boss into the ice pools. Wait for it to be stood over the pool or about to charge over the pool and shoot out the ice. Once it’s dropped in and frozen go to town shooting it in the back. The best weapon I found for this was the Gnasher. You can do massive amounts of damage with some active rounds up close.
There will be 3 occasions during the fight when Kait is transported into Swarm world. During these times just hang around the nearest ice pool and wait for the boss to approach you, as soon as they’re over the pool shoot out the ice.
The boss will also throw spikes from time to time make sure to roll clear of these as they can damage you a lot and open you up to being charged and killed instantly.
With these tips you shouldn’t have any trouble clearing this boss fight swiftly.