I play Agression (Action: Each player must sacrifice a unit, if able.)

Darth Vader has (Reaction: after you play a Sith event card, deal 1 damage to a target unit)

Does the reaction go off first before we each have to do the action?

It makes a difference cause I may kill a small unit with Vader prior to an opponent sacrificing a unit.

sorry if dumb question and thanks for answers.

I believe the played event card would need to be fully resolved to satisfy the triggering condition of playing an event card.

From the Official FAQ Page 7, Column Right, " REACTION ABILITY TIMING ", 3rd Paragraph, first line:

"A reaction ability is always used immediately after its

Thank you. Makes sense.