Looks like someone is watching my struggle against Brencis very closely. They've sent me a schematic describing how to create a unique set of armour.
Receive the letter, then choose which relic to sacrifice
The quest can begin when a randomly appearing character delivers a letter after Coen reaches Infamy level 4; Anca can also introduce the armour request. The St. Tyna’s Grove map marker and Briar Sloughs journal label are useful references, but they are not a fixed letter-delivery point.
The armour request takes one saint relic as a component, and the selected item is consumed. Four alternatives are documented: Saint’s Tibia, The Relic of St. Tyna, The Relic of St. Tobias and The Relic of St. Dasius. Compare their roles before the hand-in; the saint relic guide also lists the other required pieces, Large Lizard’s Moult, Silk Threads and Flask of Quicksilver.
Take the four components back to Nareh, the Uriashi blacksmith at Uriashi Hermitage. Tell her you have the materials, choose the relic you intend to surrender and prepare for the binding ritual. After the fight, speak to her again to receive the enhanced armour, boots, bracers and shin guards. The St. Tyna’s Grove quest marker is not the delivery location.