Brother Corhyn Questline

Quest Step	Location (Click for Map Link)
Speak to Corhyn in the Roundtable after you reach Altus Plateau and he will tell you he is going on a journey to find the Gold mask
Corhyn will move to Altus Plateau next to the Map fragment location near Altus Highway Junction site of grace
Next, you need to find Goldmask, he can be found in the north of Altus Plateau on the large broken bridge, you can use the waygate at Altus Highway Junction to get to him quickly Interact with him here and then return to Corhyn and tell him about the Goldmask After resting Corhyn will move to Goldmask’s side, speak to both of them here.
Next, you need to learn the Law of Regression Incantation by finding the Golden Order Principia in Erdtree Sanctuary To get this item you need to follow the branches up after defeating Godfrey (Golden version), when you make it up onto the balcony above the boss room head outside and turn left, jump onto the roof and through the window, follow the branch until you are stood in front of a hanging chair with a corpse in it, the Principia is here. Take the book to either Corhyn or Miriel and learn Law of Regression (37 Int Required)
Corhyn and Goldmask can now be found near the West Capital Rampart, go under the big roots and towards the Colosseum building but turn off to the left about halfway up the path to the Colosseum. Speak to both of them here.
Travel back to the Erdtree sanctuary and head out the West door, follow the path around until you reach the elevator, take it down and follow the stairs until you are in front of the large statue, use the Law of Regression here to reveal who Radagon is
Return to Corhyn and Goldmask and speak to them, tell Goldmask who Radagon is and speak to Corhyn again
Next you need to travel to the bridge near Stargazers Ruins, speak to them both on this Bridge If you have the Tonic of Forgetfulness (Rya Questline) you can try and give it to Corhyn here, he won’t accept it but it will change what happens to him
After completing Farum Azula you can find Goldmask at the bottom of the cliffs in front of the Colosseum, here you can pick up the Mending Rune of Perfect order and Goldmask Armor set if you reload the Area.
IF you didn’t have the Tonic Corhyn can be found at the base of the Large spear in the Capital of Ash, speak to him and reload the area to receive his bell bearing and Corhyn’s Robe
If you tried to give him the tonic, Corhyn will remain on the bridge near Stargazer Ruins, talk to him and reload the area to receive the same items