Spark Harvest
Sorcery
As an additional cost to cast this spell, sacrifice a creature or pay 
.
Destroy target creature or planeswalker.
With the harvest of Domri's spark, the Elderspell began to fuel Bolas's ascension to godhood.
Kieran Yanner
Commander Legends (CMR)
• #150Rulings
You must sacrifice exactly one creature or pay an extra {3}{B} to cast this spell; you can't cast it without paying one of those costs, and you can't pay either multiple times. Players can respond to Spark Harvest only after it's been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to stop you from casting this spell or to make it cost more mana.
(wotc • 2020-11-10)
Rulings
You must sacrifice exactly one creature or pay an extra {3}{B} to cast this spell; you can't cast it without paying one of those costs, and you can't pay either multiple times. Players can respond to Spark Harvest only after it's been cast and all its costs have been paid. No one can try to destroy the creature you sacrificed to stop you from casting this spell or to make it cost more mana.
(wotc • 2020-11-10)