Indestructible is a keyword ability defined in CR 702.12b: a permanent with indestructible cannot be destroyed — meaning it is not put into the graveyard by effects that use the word 'destroy' or by lethal damage. It has no effect on sacrifice.
Sacrifice is a specific game action defined in CR 701.17a: 'To sacrifice a permanent, its controller moves it from the battlefield directly to its owner's graveyard.' This action bypasses the indestructible replacement effect entirely because it does not involve 'destroying' the permanent.
When a spell or ability requires you to sacrifice a creature as a cost (CR 118.12), you simply move that creature from the battlefield to the graveyard. Indestructible provides zero protection against this, and the sacrifice is completed normally.
Concrete example: You control a Darksteel Myr (which has indestructible) and you cast Fling, which reads 'As an additional cost to cast this spell, sacrifice a creature.' You may legally sacrifice Darksteel Myr to pay that cost — it goes to the graveyard immediately, and Fling resolves dealing damage equal to its power.
Remember: indestructible protects against 'destroy' effects and lethal damage only. It does not protect against being sacrificed, exiled, or bounced.
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