Illicit Masquerade's last ability affects all creatures you control with impostor counters on them, not just ones that had impostor counters put on them with Illicit Masquerade's second ability.
When this enchantment enters, put an impostor counter on each creature you control.
Whenever a creature you control with an impostor counter on it dies, exile it. Return up to one other target creature card from your graveyard to the battlefield.
