Each time Mohg chants Nihil, you'll get bloodloss, so either heal in time with his chanting or use the Purifying Crystal Tear in your Flask of Wondrous Physick to nullify the curse. You get this by defeating Violet Bloody Finger Eleonora in the Second Church of Marika on the Altus Plate...
There are lots of great items to find in Elden Ring, chief among them is the Flask of Wondrous Physick. This item acts as a sort of Sacred Flask that you can alter using tears, giving you a single-use drink that recharges whenever you rest. You may learn about it from a note, an...