This is the first Caldron you’ll come across in the game after you talk to Dral in Mother’s Crown. The cauldrons act as places you can upgrade your override abilities so you’ll be able to override more types of machine, and also as challenging areas that you can test your skills in. The entrance to Sigma isn’t far from Mother’s Crown, just follow your waypoint until you reach the group of Watchers and Scrappers that are patrolling outside. Either kill or avoid them and head for the triangular door and Override it to gain access.
Head inside and follow the corridors, keeping an eye out for patrolling Watchers. There’s only one way to go, so you don’t need to worry about getting lost. Keep going until you reach a more open area where Aloy will notice a glowing red terminal. Go up to it and override it to create a bridge across the gap to the other side.
Once across, keep following the corridors until you get to another door that you can override. Do so and head through. You’ll see a bunch of crates being moved along above you, with glowing yellow handles on the bottom, which means you can grab a hold of them. Go to the spot underneath where the crates move past then jump up and grab one as it reaches you, then hang on for the ride. Eventually Aloy will drop off of the crate of her own accord, and now it’s up to you to navigate to the Heart of the Cauldron.
There will be crates moving across the area as you move forward. Grab hold of the first one you get to to transfer yourself over to the left hand side of the area then make your way forward. There are multiple Watchers patrolling here, as well as a new enemy type called Shellbacks. These are spider like machines that have shields and guns on their front limbs, and so are tricky in a head on fight. If you’re going to take them on, the best way is to sneak up on them and fire a Tearblast arrow. They’ve got a lot of components to knock off, and it’ll normally kill them with one well placed arrow.
Keep following your waypoint to the Heart until you get to another bridge node you can Override. Do so, and follow the paths to a part where you can rappel down into a cliffside area outside of the Cauldron. Climb up onto the triangular piece beside the cliffwall, then do a running jump to the other side, using the yellow highlighted handholds to catch yourself. Once you’re on that side, head through to the Heart.
Rappel down to ground level then use your Focus to identify all the Watchers who are already patrolling the area, this will make the upcoming pseudo-Boss fight easier. Once they’re down, climb up one of the towers and override it to disable the forcefield in the centre. This will free the Bellowback and also unleash 3 more Watchers, so prepare yourself for a tricky fight.
The most effective tactic is to concentrate on the Watchers first as they’ll just keep distracting and damaging you otherwise. Kill them all as quickly as possible then turn your attention to the Bellowback. These guys are big, and tougher than you’d think. They have 3 main attacks, a ranged exploding fireball, flamebreath and a charge. Your best bet is to stay on the move and use your tripcaster to put down explosive and shock wire traps to try to catch it out as it moves around to retarget you. If you manage to stun it, immediately hammer its weakpoints, either the back or the gullet, to get explosions and lots of extra damage. Keep putting down traps and avoiding it as best you can and soon enough the beast will go down.
When it’s done, loot the place then Override the central console to not only unlock the extra Overrride codes, but also to get taken back up to the entrance and the completion of this area.