I like the idea. Add some colorless to the cost otherwise this card is way too powerful. Also white, blue, and black never do damage. But, black does allow loss of life so I think that could be an appropriate change. I don't see anything that would make this card white for any reason, if there is I would like to understand why. Maybe make the life loss higher than 10, otherwise I choose myself and cast Emrykul on turn four. Wow I'm nitpicking and I'm sorry but it should be "at the beginning of their next upkeep" not "by the beginning..." otherwise they could pay it whenever and that doesn't fit well with how the colors work or Magic mechanics. Other than that I really like this card.

Originally, it was "... or he or she loses the game," but I thought that perhaps it was too good that way. However, you make a good point about Emrakul or Progenitus or any other big expensive card, so I think I might change it back to make it less likely to be used on yourself. That, or make it "or his or her life total becomes 5." It's white because white imposes its beliefs and views on others, and is the color representing law. Yes, it would make sense in just blue and black, but I personally like the addition of white. I made it so cheap both because it's three colors and because it has a high potential to be useless, particularly as the game lengthens and your opponent has less cards in hand and more resources.