While the Academics' Red Lump can come in handy for reducing your posture damage at the expense of losing the power to resurrect yourself, the reward for this quest isn't so much material. Instead, it's the way it dramatically expands on the universe of Sekiro and perfectly compliments FromSoftware's existing body of work. Sekiro is a game that explores themes of greed, the pursuit of power, and the desire to be immortal, and it shows how those desires, when warped, will corrupt a person into a pale imitation of their former self. Several other characters mirror Doujun's descent into insanity throughout Sekiro's events. Ultimately, you're not doing the "right" thing, so your material reward shouldn't be particularly significant. Instead, the prize is further exploring the game's themes and seeing the full spectrum of FromSoftware's artistic vision.