In Magic, there is a “Golden rule” that you should always remember. Basically, it says that “can’t” overrides “can”.
So lets explore what happens when Narcissa controls Havoc Festival, and Annabelle casts Sphinx’s Revelation. Sphinx’s Revelation says to gain X life, while Havoc Festival says players can’t gain life.
Can’t > Can.
No life will be gained. But cards will still be drawn.
Now, “why doesn’t this stop the card draw?” you might ask.
It’s because there are two instructions (indicated by two verbs) which are independent. It doesn’t matter that they’re in one sentence. Sphinx’s Revelation does not state “Gain X life; if you do, draw X cards” or “Gain X life, then draw cards equal to the amount of life you just gained” (in either case, yes, Havoc Festival would stop cards from being drawn). There is no link between the drawing cards effect and the gaining life effect, other than the fact that both the numbers are equal to X. One effect not happening will not stop the other, since they are not conditional on one another.
Note that some cards do link two instructions, making the second effect conditional on the first, but those cards will make that explicit in their text (e.g., “for each creature destroyed this way” or “for each permanent put into a graveyard this way” or “for each damage prevented this way” on cards like Decree of Pain, Terastodon, or Swans of Bryn Argoll).
Today’s Rules Tip written by Eddie Cheung (welcome to the team!)