Game your life with the Grim Reaper. If you win, your health and mana is refilled. If you don't, your maximum health is decreased by 25%.
The chance of winning is 70%.
As of the current version (September 2015), supposedly due to programming error (multiplying by 3/2 instead of 3/4), losing in fact increases your max health by half without any negative consequences.
• Usually only a good idea when you are really about to die.