While resolving Fandaniel's second ability, the next opponent in turn order chooses up to one nontoken creature to be sacrificed, then each other opponent in turn order does the same. Then each of the chosen creatures are sacrificed simultaneously. Finally, each opponent who didn't sacrifice a creature this way loses 2 life for each instant and sorcery card in your graveyard.

Fandaniel's first ability resolves before the spell that caused it to trigger. It resolves even if that spell is countered or otherwise leaves the stack.