You choose whether to make a copy as the demonstrate ability resolves. This happens before the original spell resolves. Your copy goes on the stack above the original spell.
Incarnation Technique
You may choose any creature card in your graveyard, not just one that you milled this way.
This means that if you cast a spell with demonstrate and both you and an opponent copy it, the opponent’s copy will resolve first, then your copy will resolve, and finally the original spell will resolve.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Alexander Mokhov
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
