Mistress of Pain deals 1 damage to the enemy hero, triggering her effect. Due to Auchenai Soulpriest, her effect is treated as her dealing 1 damage to my hero, which triggers her effect again and again and again. I was at 30 health when this occurred and just repeatedly took 1 damage until my health was reduced to -3, which obviously killed me.
I get that it technically makes sense with how the card is worded, but is it really OK for an effect to be able to trigger itself in an endless loop while both players just sit there and watch? I was under the impression that her effect would only trigger when she physically attacks and damages a hero or minion, which sounds pretty logical to me.
Even if this is an intentional mechanic for some reason, why am I dying from "overkill" damage when the effect is dealing damage in increment of 1?
Blizzard has stated before that this is ok and working as intended. It's a very specific set of circumstances and you have to set yourself up for it. It has happened more often when someone steals the succubus and learn the hard lesson that way. If you have the 1/100000 game of having the Soulpriest already down and somehow on your opponent's turn you acquire the mistress and they trigger it then congrats on slipping into a fun story.
I doubt that anyone here would have the technical knowledge t o know, but why stop at -3 and not 0?
There are some gigantic combos that stop after a while as well. I don't know the exact rules for this. I saw a video where a priest and a warrior set up the board with a couple 10,000 health minions using Cho and Divine Spirit. The warrior cast Bouncing Blades and it went crazy for about 20 seconds and then stopped bouncing well before any minions were dead. Off hand though, maybe they wanted a little buffer room just to make sure they person is dead before they stop the combo early decided 3 (or 11 in bianary) was a short enough cushion to program in. One more bit from binary and it's taking you to -7 and they thought that making it go that far would take too long.