Note: Acquire a splicer key from a Brood Mother wizard in the Plaquelands before starting this quest.
You should have a quest asking to scan an anomaly you must head to Archons Keep. There you must look for a patrol beacon that has the ghost symbol (Normally recon missions.) If it's not there return to orbit and come back until it shows up.
It will have you go towards the Archons forge. Once you follow it you it will require you to open a laser gate with the splicer key, fight a room of fallen with an elite captain and pick up the Relic (Hammer.) After that you go down some stairs and are in a big room with a SIVA Walker and some enemies. Clear our the enemies and avoid the Walker. It is entirely possible you don't have to defeat the Walker. If you stun him then run to the right of him you can scan and leave before he comes back up.
Be very careful not to go into the hole that's next to the medallion before scanning it. You'll have to return to orbit and possibly restart the quest.
After completing this step you have to speak to Tyra Karn. She then asks you to find more Iron Medallions. There is one in Felwinter peak and one in every patrol area of the Plaguelands. I'll post screenshots in a little while of the locations. They are not hard to find because your ghost points them out for you, but if you're having a hard time locating them you can ADS with a sniper.
Once you've completed that you go to Banner Fall and run around scanning archives, a library and blueprints. Once that is complete the towers defense systems go off and alert you that you're under attack by Fallen.
At this point a bomb is put on one side of the map and you must defend the area. It's a lot like defending a Warsat but much smaller area and a lot of exploding shanks. Once the ghost disarms the bomb you must do it again on the opposite side of Banner Fall. After both the bombs are done you fight a giant fallen captain with the help of the banner Fall defense system.
You then return to the cryptarch and she asks you to find 5 pieces of SIVA cluster. Use destinysivaclusters.com/
After that you are asked to go back to Earth and find the pieces to create the ghorn. It starts in the Steppes as you clear enemy Fallen and dodge slow mines and it ends up on the Frontier map from crucible.
Here your ghost is in a room with Ghorn trying to reconstruct it while you fight off waves of fallen with the occasional elite captain with a scorch cannon.
If you thought defending the bombs was challenging this level has many more enemies coming at you from different sides. It's fun and intense to do solo if you're looking for a challenge, otherwise go with a fireteam.
After fighting them for what feels like forever you receive the Y1 Ghorn at 350 LL. At this point you can leave it like that or infuse it into the Iron Ghorn. Immediately after you are ordered to clear out the remaining enemies, to so this they spawn in a ton of heavy ammo crates so you can go wild with your shiny new rocket launcher. You'll have to fight off some SIVA Fallen dregs, walkers, and dregs on spikes. Easy pickings for a Ghorn.