Thanks to GreenThePyro for sharing this
Warning: this is missable and it's be impossible to finish if Leshy/Heket/Kallamar/Shamura leave your cult.
The Relics of the Old Faith added... "Relics", there are 37 relics, and 13 of them are unlock as soon as you encounter Chemach the new npc, for the first time, after defeating Leshy she can show up in a crusade, or if you beat the game already, after entering the new purged darkwood forest, there are more 13 relics from sermon upgrades, so, heres a list of what you need and how to get them, i'll do them in order.
Eye of Leshy
-After defeating Leshy, and having him in your cult, he will give you a task to find his eye in Darkwood forest.
Blessed Clauneck's Hair
-From the Sermon upgrade "Blessings of the Relics"
Damned Clauneck's Hair
-From the Sermon upgrade "Damnation of the Relics"
Clauneck's Hair
-Unlocked at the start after talking with Chemach
Galvar's Charm
-Unlocked at the start after talking with Chemach
Blessed Laplace's Dice
-From the Sermon upgrade "Blessings of the Relics"
Damned Laplace's Dice
-From the Sermon upgrade "Damnation of the Relics"
Laplace's Dice
-Unlocked at the start after talking with Chemach
Beak of the Fowler
-Fishing
Toe of the Icegore
-Unlocked at the start after talking with Chemach
Eye of Icegore
-From the Sermon upgrade "Eyes of the Lost Relics"
Seal of the Bishops
-Unlocked at the start after talking with Chemach
Shamura's Skull
-After defeating Shamura, and having them in your cult, they will give you a task to find their skull in Silk Cradle. (when indoctrinated, Shamura will be rebelling, so take care of that, or they will leave, making this impossible)
Blessed Tears of the Vengeful Father
-After returning Baal to Forneus
Damned Tears of the Merciful Mother
-After returning Aym to Forneus
Heket's Throat
-After defeating Heket, and having her in your cult, she will give you a task to find her throat in Anura.
Blessed Ala's Teeth
-From the Sermon upgrade "Blessings of the Relics"
Damned Ala's Teeth
-From the Sermon upgrade "Damnation of the Relics"
Ala's Teeth
-Unlocked at the start after talking with Chemach
Tail of the Slythor
-Unlocked at the start after talking with Chemach
Eye of the Slythor
-From the Sermon upgrade "Eyes of the Lost Relics"
Clauneck's Shoe
-After speaking with Clauneck, he will give you a quest, you need to go behind the lighthouse
The Transmogricon
-Killing a critter in a crusade.
Kudaii's Toenail
-Unlocked at the start after talking with Chemach
Kudaii's Fingernail
-After speaking with Kudaii, he will give you a quest, you need to go in front of the fish in Smuggler's Sanctuary and tickle him.
Eye of the Fervant
-From the Sermon upgrade "Eyes of the Lost Relics"
Shunrue's Charm
-Unlocked at the start after talking with Chemach
Stihi's Tongue
-Unlocked at the start after talking with Chemach
Kallamar's Ear
-After defeating Kallamar, and having him in your cult, he will give you a task to find his ear in Anchordeep.
Warracka's Right Fist
-Harvesting meat from followers.
Blessed Exorcist's Finger
-From the Sermon upgrade "Blessings of the Relics"
Damned Exorcist's Finger
-From the Sermon upgrade "Damnation of the Relics"
Exorcist's Finger
-Unlocked at the start after talking with Chemach
Turua's Tentacle
-Unlocked at the start after talking with Chemach
Blessed Aethrile's Powder Box
-From the Sermon upgrade "Blessings of the Relics"
Damned Aethrile's Powder Box
-From the Sermon upgrade "Damnation of the Relics"
Aethrile's Powder Box
-Unlocked at the start after talking with Chemach