2006-05-01 : You can choose an X that’s equal to your life total. If you do, you’ll pay that much life when the spell is cast and will lose the game before it resolves.

2006-05-01 : You can’t choose an X that’s greater than your life total.

2013-04-15 : You must pay both X life and {X}{B}. For example, if you want each other player to lose 2 life when Bond of Agony resolves, you must pay {2}{B} and 2 life.