If you want a chance against The Harvester, find these. They'll upgrade your golem. You'll have to do a fair amount of backtracking to get all six, but they're all either in plain sight or just off the main path. There are four types, Healing, Mechanics, Fire, and Lightning. Fire and Lightning give extra attacks to your golem. Healing allows it to heal your squad. Mechanics toughens up your golem. Find them all. They'll help out a ton.
Each note also requires a different lyrium switch to be flipped. Each switch corresponds to a different color (blue, purple, white, green, and red). Each color allows you to access doors or chests that are encased in the mist of the same color. The locations of each note can be found in this thread. Credit goes to Method for the list.