The Philosopher’s Stone Achievement
The Philosopher’s Stone15
Aid Essex Mast on his questDuring the Werewolf's chapter, you will automatically be given a side-quest called "Trapping Souls" that will unlock this achievement. Make sure when you get this, you ignore it for a moment and head to Grackle first to complete the Loyalty to the Pack (15G) achievement. Doing this achievement first will void the other one.
To complete this quest, go to Oleander Temple and you'll find a button on the wall behind a giant incense burner. Inside the next room you'll find the Ancient Soul Vessel. Grab that and head out of the temple to find Essex waiting. Hand over the item to finish the quest and unlock the achievement.
See Eternity Can Waits (10G) for info on keeping Essex Mast alive throughout the game.