Cauldron Sigma is the first Sigma in Horizon Zero Dawn. Cauldrons contain data that will allow you to override more machines. The quest for Cauldron Sigma and its location can be started when you arrive at Mother's Crown during the A Seeker At The Gates main quest line. Talk to Dral, a woman with a scar on her face, standing by Nora tribesmen who are injured. She will have a green exclamation point over her head. She will tell you all about Cauldron Sigma.
Travel to the location of the Sigma Cauldron by setting the quest as active and then follow your quest marker. At the entrance of the Cauldron there will be machines around the area. Take them out or sneak through to the entrance.
Upon entering the Cauldron you must rappel down and then follow the path ahead. You will encounter some Watchers but you can shoot the triangular steam vents to provide you some cover. Navigate your way through the cauldron by jumping onto platforms and climbing up walls.
As you get closer to the cauldron core you will start to encounter Shell-Walkers. At this point of the game it would be best to just avoid them and run away from them.
Once you reach the cauldron core room there will be a Fire Bellowback in the center of it with an electric shield protecting it. Climb up one of the pillars and override it to bring down the shield. This will start a boss fight with the Fire Bellowback. Kill the Fire Bellowback and in the center of the room the Cauldron Core will appear. Override it to complete the quest.
