Brood Mother is a ‘Legendary Hunt’ in Borderlands 3. It’s a rare spawn, so she won’t be there every time you load the map. Luckily there is a save station nearby, that way you can easily save&quit and jump right back in.
The Brood Mother is located in The Pyre of Stars on the planet Nekrotafeyo. You do have to navigate the map all the way around in order to get to her.
You can farm Brood Mother for these legendary weapons as it is the dedicated loot source for these items. Brood Mother has a 30% legendary drop chance.