World at War

If a replacement effect would cause a spell with rebound that you cast from your hand to be put somewhere else instead of your graveyard (such as Leyline of the Void might), you choose whether to apply the rebound effect or the other effect as the spell resolves.

If you cast a card from exile this way, it will go to your graveyard when it resolves, fails to resolve, or is countered. It won’t go back to exile.

Rebound will have no effect on copies of spells because you don’t cast them from your hand.

If you cast a spell with rebound from anywhere other than your hand (such as from your graveyard due to Sins of the Past, from your library due to cascade, or from your opponent’s hand due to Sen Triplets), rebound won’t have any effect. If you do cast it from your hand, rebound will work regardless of whether you paid its mana cost (for example, if you cast it from your hand due to Maelstrom Archangel).

Multiple World at War effects are cumulative, as long as they’re cast early enough. The first one modifies the turn structure to this: beginning phase, precombat main phase, combat phase, postcombat main phase, [new combat phase], [new postcombat main phase], ending phase. Each subsequent one inserts another combat phase and main phase into the turn after the original postcombat main phase and before the newest combat phase.

Unlike other similar cards (such as Relentless Assault), World at War doesn’t untap the creatures that have attacked this turn when it resolves. Rather, those creatures untap when the new combat phase created by the spell begins. All creatures that attacked this turn untap, regardless of which combat phase they attacked in (if there have been more than one) or who controls them (if you’re playing a Two-Headed Giant game, for example).