The Blackthorn Key by Kevin Sands
Synopsis from Goodreads:
“Tell no one what I’ve given you.”
Until he got that cryptic warning, Christopher Rowe was happy, learning how to solve complex codes and puzzles and creating powerful medicines, potions, and weapons as an apprentice to Master Benedict Blackthorn—with maybe an explosion or two along the way.
But when a mysterious cult begins to prey on London’s apothecaries, the trail of murders grows closer and closer to Blackthorn’s shop. With time running out, Christopher must use every skill he’s learned to discover the key to a terrible secret with the power to tear the world apart.