The card will return to the battlefield under your control only if it’s still in the graveyard when Unhallowed Pact’s ability resolves. If it’s not (perhaps because an ability like undying has already returned it to the battlefield), nothing happens.
2012-05-01
If Unhallowed Pact is enchanting a token creature, that creature can’t return to the battlefield.
Enchant creature
When enchanted creature dies, return that card to the battlefield under your control.
