Watch for side effects
Notice! This mission will be available only after you complete Path of Knowledge!
Meet with Neloth in Tel Mithryn and ask him about his latest work. The mage will offer you to test out his latest spell - which is supposed to increase the vitality of the enchanted person. Of course there's always a risk that it won't work exactly as intended, but what would be science without risk (as a certain GLaDOS liked to think)?
Let the mage cast his spell on you and you will receive a permanent bonus of 25 point to your health. Don't be happy too soon - if you get any contact with water, you will receive an identical penalty. Return to Neloth for help!
Talk to Neloth
Return to the tower in Tel Mithryn and tell the mage of the side effects that you experienced. Neloth will moan a bit, but eventually take off the spell and pay for your trouble (as an old miser, he will only give you a few coins). Even though the spell turned out to be completely useless, it was still worth the try, right?