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Overview
When you cast Emrakul, the World Anew its "When you cast this spell" ability triggers and will change control of the targeted player's creatures when that triggered ability resolves . Casting or resolving Heroic Intervention is an instant that gives permanents you control hexproof and indestructible until end of turn; that continuous effect (from the resolved instant) can prevent Emrakul from being destroyed, which means Emrakul won’t leave the battlefield and therefore its "When Emrakul leaves the battlefield, sacrifice all creatures you control" trigger will not occur in that case . If Emrakul leaves the battlefield for a reason other than destruction (bounce, exile, sacrifice, etc.), Heroic Intervention’s indestructible won’t stop that and the leave-the-battlefield trigger will occur and, when it resolves, you will be required to sacrifice the creatures you then control .
Step 1
You announce you are casting Emrakul, the World Anew; Emrakul is put on the stack and you choose the target player for its "When you cast this spell" trigger as part of casting the spell .
Step 2
The "When you cast this spell, gain control of all creatures target player controls" triggered ability is created on the stack (it is a triggered ability that triggered when you cast the spell) and will go on the stack and resolve later .
Step 3
Heroic Intervention is cast and resolves (if you cast it at an appropriate time); when Heroic Intervention resolves it creates a continuous effect that gives permanents you control hexproof and indestructible until end of turn .
Step 4
If an effect would try to destroy Emrakul while it has indestructible from Heroic Intervention, it won’t be destroyed; because it doesn’t leave the battlefield the "When Emrakul leaves the battlefield" ability doesn’t trigger in that case .
Step 5
If Emrakul nevertheless leaves the battlefield for any other reason (exile, bounce, sacrifice, etc.), that event causes Emrakul’s leave-the-battlefield triggered ability to be put on the stack (it triggers on the leave event) and when that triggered ability resolves you must sacrifice all creatures you control at that time (you only sacrifice creatures you currently control) .
Step 6
If Emrakul was cast via Madness (or another alternative cost), casting still follows the normal casting steps; see the madness ruling for timing if the discard happened during resolution of another spell or ability .
Resolution
Heroic Intervention can save Emrakul, the World Anew from being destroyed (because indestructible prevents destruction), which prevents Emrakul from leaving the battlefield and so prevents its leave-the-battlefield triggered ability from triggering; but Heroic Intervention does not prevent Emrakul from leaving the battlefield for non-destruction causes (bounce, exile, sacrifice), and if Emrakul leaves the battlefield for any reason the triggered ability "When Emrakul leaves the battlefield, sacrifice all creatures you control" will be placed on the stack and will cause you to sacrifice the creatures you control when it resolves .
