If you wish to learn Miracles in Dark Souls 3, Irina of Carim is the NPC merchant who will sell most of them to you. You can find Irina in the Undead Settlement, but first you need to obtain the Grave Key. Our walkthrough of the side areas in the Undead Settlement covers exactly how to find the Grave Key and get Irina to become a merchant in the Firelink Shrine.
For a quick rundown of the Grave Key, you need the Mortician’s Ashes from the area where the giant bombards you with arrows. Once you have this item in hand, give it to the Shrine Handmaid in Firelink and she will add the Grave Key to her inventory for 1,500 souls. Once you have the key you can go down to the sewers in the Undead Settlement where you fought the rats and make your way around to Irina, freeing her from the cell.
When Irina heads back to the Firelink Shrine, she becomes a Miracle merchant. If you bring her various Braille Tomes she will learn new Miracles to teach you. However, there is a price to be paid for giving her certain tomes. If you give Irina the Deep Tome or Londor Tome, then purchase any of the resulting Dark Miracles from her, she falls into darkness and will be kidnapped. If you buy every Dark Miracle from her, she stops selling and you will only be able to get sad dialogue from her (even worse if you’re wearing Eygon’s gauntlets when you speak to her).
If you truly wish to help this blind nun, buy only holy Miracles from her inventory and she will eventually become a Fire Keeper and move to the bottom of the tower behind the Firelink Shrine. The fastest way to access the bottom of the tower is to head out the main entrance (keep rolling into the fog if it’s present and it will dissipate), then go right and circle around to the gate. This will only be open if you’ve already unlocked Unbreakable Patches as a merchant.
Allowing Irina to become a Fire Keeper causes any Dark Miracles tomes to be left behind at her previous location in Firelink Shrine. When this happens you can give them to Karla the Witch who will learn and teach them to you without issue.
For more information on the various NPCs, head over to our Dark Souls 3 walkthrough and guide.