I found the text on the card rather strange:

Instead of "sacrifice a unit to corrupt target unit" the card reads "when you sacrifice a unit, corrupt target unit."

Am I right to assume that the sacrifical is not meant to be a cost, but a requirement that has to be fulfilled to trigger Sacrifical Pyre? If that is the case, I would have to sacrifice a unit by other means (like using Slave Pen action "sacrifice a unit to put a resource token on this card") to use Pyre´s ability.

As I am not a native speaker I could be wrong about some semantic or grammatical nuance here, though… Sorry for the weird question. I just want to interprete the card´s effect the right way

You are correct. It's a triggered action, the trigger being the sacrifice of a unit.

Thank you very, very much for the fast answer.