I've been casting masochism on Nightmare and the spell states that for every 5 damage he gains 1 AP and 1 Dmg permanently. When the spell wears off Nightmare loses damage. He goes from 12 to 8 damage. He's such a great target for masochism too since he self heals.
I think its a glitch machoism suppose to gain permanent damage the spells says For 3 turns, damage to friendly champions is prevented and target owned champion loses HP equal to 2 less than the damage prevented. If target champion is a Demon, it gains 1 AP and +1 DMG permanently for every 5 damage transferred this way. This effect ends if target champion is no longer in play.
Well @badgerale and I figured out the bug. The reason why his damage was going to 0 was because of the heals coming from the lava when a fire eater would walk over the lava. So the damage bonus that was coming from masochism would happen in reverse when getting fire eater heals. It doesn't happen from regular heals. The masochism target must be a demon for this to occur.