This is a quest from Tirso in Albero. He's the guy on the level you start on when you run into the main building. For his quest you need to bring him 6 healing herbs. They are in:
- Albero: but you need to find a teleporter in a different area and teleport back to Albero (the room it's in unlocks from the inside), the item is in the teleporter room
- Jondo: at the very bottom right room of the area
- Where Olive Trees Wither: go into the area after Gemino, and all the way across to the left hand side of the area, at the bottom left go through the opening and it's in there
- Sleeping Canvasses: in the tall room with the ladder in the middle and the 3 possessed paintings
- Patio of the Silent Steps: in the second to last "room"
- Mountains of the Endless Dusk: coming from the Brotherhood of the Silent Sorrow, on the left, exit the area with the Prie Dieu and it's in this area
The trick seems to be that you need to bring each one back to Tirso before you kill your next boss, so it's best to return to Albero to turn a herb in before you carry on, even if it is a bit annoying back tracking. You then need to progress to/kill Quirce in the Wall of Holy Prohibitions and talk to Tirso again, he should give you a rosary knot and the achievement will pop.