Another quest from the ever-present Neloth . This time you’ll want to her him ask about advancing the art of magic for this quest to begin. It’s a fairly simple and short quest in return for some easy gold.
Simply agree to being his test subject and then once you’ve gained your health boost, immediately go walk into the nearest pool of water (won’t be far away) and your health will be reduced by 25% permanently. Head back to Neloth and he’ll reverse this and give you your money in return for your help.