Horizon Zero Dawn's To Curse the Darkness should be tackled when you're high enough level, ideally following the events of The Sun Shall Fall after the main quest branched, getting you back to the main story of Aloy and the history of her world.
Your main task here is to take down the Eclipse Network, which is helping them communicated via a chain of Focus devices and identify you on sight. Crashing it, though, won't be easy amidst heavy guard presence from the Cultists.
If you're after help for other parts of the game, consult our complete Horizon Zero Dawn walkthrough and guide.
Infiltrate the Eclipse Base
No more detective work for you now - it's time to do some real damage to the real foe, and take down the Eclipse Network run by the true evil here, known as HADES. Here's how:
Go to the Location that Sylens indicated - Faster with a mount, as this is into territory the story won't have taken you yet.
Climb the Escarpment - After you get there you'll need to head to a cliff face nearby, and climb up using the yellow-marked indicators on the rocks as usual.
Infiltrate the Eclipse Base - Once you've climbed up, Sylens appears to you with a talk about the plan. It's time to get into the Eclipse Base, which happens to be guarded by waves of corrupted machines. You can take them down (best to do it one at a time, stealthily) or sneak past the entire lot with a bit of ingenuity, as there are lots of tall grassy areas to hide in. It's up to you!
Search for the derelict Tallneck and Crash the Eclipse Focus Network
Search for the derelict Tallneck - Once you've reached the base proper, it's time to find the derelict Tallneck that's sending out the signal for the Eclipse Network.
Do so by sneaking through the enemies, across the bridge, and into the tent that houses the ceremonial armour. Just to the left is a wonky tree that you then need to push over, by interacting with Triangle, to get across the ravine.
Crash the Eclipse Focus Network - Follow the path along the cliffs to a wooden structure which takes you up the Tallneck. At the top, there's a cutscene as you take the Network down.
For more tips, explainers and advice on Horizon be sure to check out our main Horizon Zero Dawn guide and walkthrough hub, which includes advice for things like Horizon Zero Dawn's override Cauldrons and which machines they let you control, getting hold of the Golden Fast Travel Pack for unlimited fast travel, finding and unlocking the very powerful Horizon Zero Dawn Shield Weave Armour and Power Cell locations, and then for DLC players there's our main Horizon Zero Dawn Frozen Wilds walkthrough and guide, a page on how to earn Bluegleam quickly, and finally, a page on the handy A Secret Shared quest and how to upgrade your spear in Horizon Frozen Wilds.
Escape the Eclipse Base - This is an old school run-for-it style escape. Don't fight anyone, just run from waypoint to waypoint, like you did during the Proving.
As long as you don't slow down too much, you won't get killed and can make a clean escape. Your finish line is the bridge back at the camp, which on the left hand side has a plank you can dive off and rappel from to get clear.
After that spectacular escape, it's best to get straight on to the next quest: Deep Secrets of the Earth.