Intervene

2004-10-04
If a spell is modal, use the target(s) of the current mode only for this check. Targets for the other modes are never selected.

2004-10-04
Intervene can only target a spell that is currently targeting a creature.

2004-10-04
It means “targeting at least one creature” not “exactly one creature”.

2004-10-04
If the spell’s target leaves the battlefield before this spell resolves, Intervene won’t resolve since it no longer targets a spell that targets a creature.