It turns out that seeing the faces of foes past in the Underworld is the least of the heroes' worries on Once Upon a Time. Emma (Jennifer Morrison), Regina (Lana Parrilla) and crew will have to go toe to toe with the Lord of the Underworld, Hades (Greg Germann), if they want to bring Hook (Colin O'Donoghue) back to the land of the living with them. And he may be their toughest adversary yet.

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"He's probably the most powerful villain we've seen to date," Parrilla tells TVGuide.com. "He's the God of Death. If he decides that you're dead, you're dead. He's that powerful. It's a really dangerous place to be."

The heroes don't have long to be playing chess with Hades either. The longer they stay in the Underworld, the greater the chance of them getting stuck there. "The Underworld is not Hell. It's purgatory," Parrilla says. "You're in this limbo state. Everyone is warning them to leave because Hades wants to keep all the souls that are down there. If you don't get out quickly you can end up trapped down there as well."

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