For Holy Trinity you have to equip the Kusanagi Tsurugi, Yasakani Magatama, and Yata Mirror.
1.Kusanagi Tsurugi - Drop by Yamata-no-Orochi (3rd boss) in the mission The Demon King Revealed in the Omi Region. Smithing Text can also be found in Yagyu Sekishusai in the mission The Two Masters in the Tokai Region on way of the strong or demon difficulty.
2.Yasakani Magatama (accessories) - Can be found in THE TRAIL OF THE MASTER in Tokai Region on way of the strong or demon difficulty.
3.YATA MIRROR (ACCESSORIES) - CAN BE FOUND IN THE TRAIL OF THE MASTER AT TOKAI REGION (LEVEL 765) ON WAY OF THE WISE DIFFICULTY.
Farming method - see the videos 2nd part - start the misson the trail of the master at tokai region (level 765) on way of the wise difficulty. Go to 3rd level or top level to drop on the slim ledges. Then go the secret room on the right side. First open the package if you don’t find it then open the door use the shrine again open the package if you don’t find it then take the left item not the accessories then use the shrine again open the package if you don’t find it again then use Himorogi Branch to go to select or main Manu. Try again until you find yata mirror or Yasakani Magatama.
Note - It took me 100+ try to find Yata mirror in the trail of the master at tokai region (level 765) on way of the wise difficulty. So just keep on trying until you find yata mirror accessories.
Links -
nioh.wiki.fextralife.com/Kusan...