When you first land on Nessus, you'll be given the task to investigate the Cabal. Jump down and head down the path where you'll find the Vex battling off the Cabal forces. You'll encounter a shielded Minotaur guarding the small tower near the edge of the battlefield. Defeat all the enemies including the shielded Minotaur and use your Ghost to hack the small tower.
A series of floating rock platforms will materialize and use them to reach the portal that will take you across the milk water falls. Continue down the path and defeat the three Legionairies and the Psion, then hop into the next portal that will take you to the cave filled with Vex architecture. Follow the path into the cave and you'll eventually come across a bunch of Vex and Cabal forces fighting each other. Take them all out and watch for the Bloodguard Centurion.
It would also be good to note that at the bottom of the chasm there is a purple glowing ball that can be used as a portal that sends you upwards to the floor where more Vex will greet you with angry vigor. Defeat all the enemies and continue onward.
Once out of the cave, you'll have to jump through some platforms to reach the dig site. There will be some enemies along the way but none of them that you won't be able to handle. Some Warbeasts will be around so be careful and wary since they can be quite sneaky and can deal annoying damage. Near the end, you'll have to defeat two of the Drill Site Security Officers found inside their huts. Once you've defeated them your new objective will be to board the Cabal Drill. Activate and enter the levers where you'll be lifted up to a platform with more Cabal forces and leading you to the drill site.
The actual Digsite is where most players end up dying a lot since Guardians will have to navigate their way through the Digsite and avoid the oncoming blades of the Cabal Drill.
Once you see the dig site stick to the right and search for small pockets or rocks that will provide cover from the drill's blades. Along the way you'll encounter enemies so make quick work of them while avoiding the drill blades. You don't have to kill all the enemies and they can also die from the drill's blades should they be unfortunate enough. Continue through the dig site until you see a very deep drop leading into a tunnel. Continue further into the tunnel until you find a hole in the ground. Drop down to start the fight against Protheon, Modular Mind.
The Modular Mind will have three Phases. The first phase is fairly simple. Deal with the Vex forces that Protheon will spawn and reduce its health to about 75%. Make sure to aim for the head and when phase two is about to start Protheon will disappear and destroy the ground.
In Phase 2, Protheon will occassionally scorch the ground and it will turn red causing constant damage. Stay in the air or jump onto one of the tall pillars to avoid getting scorched to death. Protheon will again summon Vex forces to aid him, just take them out and keep the heat on Protheon. Use the pillars and stones as cover and you should be fine. After you've reduced its health to about half the Modular Mind will disappear and once again consume the floor leading you to drop into Phase 3.
In Phase 3, the Guardians will land on a smaller platform surrounded by a vat of white-bluish liquid. Stepping on the liquid will deal constant damage. At first the modular mind will wander about on the outer portion of the area, spawning more Vex troops and leaving you with very little cover to hide behind. Keep up the same strategy up until Protheon has nearly 1/4 health left. Keep aiming for its Head and if you decide to destroy its arms before the head, Protheon will be forced to take the fight to you earlier than expected making it harder to deal with. If that happens, Guardians must keep spread out as it chases one of you. This last bit can be a bit tricky since Protheon will start to spawn Vex forces more aggressively and be wary of his shockwave attack which will knock Guardians off to the liquid. Kite around and keep aiming for the Eye and you should be able to defeat him.
A chest will appear with your loot. Good luck and enjoy the spoils.
